Making a C# winform project. How can I add an additional loop with which to fill additional text boxes in textboxgroup(s)?

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WinForms

C#7.0

I have recently made a C# WinForm calculator project which fills a single 3x4 block of text boxes. Like so:

private void calculate_Click(object sender, EventArgs e)
{
    TextBox[][] textBoxOutputGroups = new[]
    {
        new [] { tb_Stress111, tb_Stress112, tb_Stress113, },
        new [] { tb_Stress121, tb_Stress122, tb_Stress123, },
        new [] { tb_Stress131, tb_Stress132, tb_Stress133, },
        new [] { tb_Stress141, tb_Stress142, tb_Stress143, },
    };

    if (TextBox1.Text == "" || TextBox2.Text == "")
    {
        MessageBox.Show("Some textboxes are empty!", "Error", 
                         MessageBoxButtons.OK, MessageBoxIcon.Error);
    }
    else
    {
        double var1;
        double var2;
        double var3;

        var1 = Convert.ToDouble(TextBox1.Text, CultureInfo.InvariantCulture);
        var2 = Convert.ToDouble(TextBox2.Text, CultureInfo.InvariantCulture);

        if (TextBox3.Text == "")
        {
            var3 = (-0.007 * var1 * 2) + 1.8;
        }

        else
        {
            var3 = Convert.ToDouble
                   (TextBox3.Text, CultureInfo.InvariantCulture);
        }

        for (int WP = 0; WP <= 3; WP++)
        {
            var S1 = Math.Pow((var2 - var1) * WP, 2);
            var S2 = Math.Pow(4 + var3, 2);
            var S3 = Math.Pow(var1, 2);

            double[] outputValues = new[]
            {
                S1 * var2 / 3,
                S1 * S3 / WP,
                var3 + var1 * S2 / WP,
            };

            for (int i = 0; i < outputValues.Length; i++)
            {
                 textBoxOutputGroups[WP][i].Text =                     
        outputValues[i].ToString(CultureInfo.CreateSpecificCulture("en-GB"));
            }
         }
    }
}

What I have tried:

What I'm trying to do now is repeat this with 11 groups of 3x4 textbox groups (which I have already created and named in the form), outputs of which would be filled and modified based on the conditions of the new loops which contain the 'j' variable also a new tadd variable would be added.

Something like this:

        private void calculate_Click(object sender, EventArgs e)
        {
            TextBox[][] textBoxOutputGroups = new[]
            {
                new [] { tb_Stress111, tb_Stress112, tb_Stress113, },
                new [] { tb_Stress121, tb_Stress122, tb_Stress123, },
                new [] { tb_Stress131, tb_Stress132, tb_Stress133, },
                new [] { tb_Stress141, tb_Stress142, tb_Stress143, },

                new [] { tb_Stress211, tb_Stress212, tb_Stress213, },
                new [] { tb_Stress221, tb_Stress222, tb_Stress223, },
                new [] { tb_Stress231, tb_Stress232, tb_Stress233, },
                new [] { tb_Stress241, tb_Stress242, tb_Stress243, },

                new [] { tb_Stress311, tb_Stress312, tb_Stress313, },
                new [] { tb_Stress321, tb_Stress322, tb_Stress323, },
                new [] { tb_Stress331, tb_Stress332, tb_Stress333, },
                new [] { tb_Stress341, tb_Stress342, tb_Stress343, },

                new [] { tb_Stress411, tb_Stress412, tb_Stress413, },
                new [] { tb_Stress421, tb_Stress422, tb_Stress423, },
                new [] { tb_Stress431, tb_Stress432, tb_Stress433, },
                new [] { tb_Stress441, tb_Stress442, tb_Stress443, },

                new [] { tb_Stress511, tb_Stress512, tb_Stress513, },
                new [] { tb_Stress521, tb_Stress522, tb_Stress523, },
                new [] { tb_Stress531, tb_Stress532, tb_Stress533, },
                new [] { tb_Stress541, tb_Stress542, tb_Stress543, },

                new [] { tb_Stress611, tb_Stress612, tb_Stress613, },
                new [] { tb_Stress621, tb_Stress622, tb_Stress623, },
                new [] { tb_Stress631, tb_Stress632, tb_Stress633, },
                new [] { tb_Stress641, tb_Stress642, tb_Stress643, },

                new [] { tb_Stress711, tb_Stress712, tb_Stress713, },
                new [] { tb_Stress721, tb_Stress722, tb_Stress723, },
                new [] { tb_Stress731, tb_Stress732, tb_Stress733, },
                new [] { tb_Stress741, tb_Stress742, tb_Stress743, },

                new [] { tb_Stress811, tb_Stress812, tb_Stress813, },
                new [] { tb_Stress821, tb_Stress822, tb_Stress823, },
                new [] { tb_Stress831, tb_Stress832, tb_Stress833, },
                new [] { tb_Stress841, tb_Stress842, tb_Stress843, },

                new [] { tb_Stress911, tb_Stress912, tb_Stress913, },
                new [] { tb_Stress921, tb_Stress922, tb_Stress923, },
                new [] { tb_Stress931, tb_Stress932, tb_Stress933, },
                new [] { tb_Stress941, tb_Stress942, tb_Stress943, },

                new [] { tb_Stress1011, tb_Stress1012, tb_Stress1013, },
                new [] { tb_Stress1021, tb_Stress1022, tb_Stress1023, },
                new [] { tb_Stress1031, tb_Stress1032, tb_Stress1033, },
                new [] { tb_Stress1041, tb_Stress1042, tb_Stress1043, },

                new [] { tb_Stress1111, tb_Stress1112, tb_Stress1113, },
                new [] { tb_Stress1121, tb_Stress1122, tb_Stress1123, },
                new [] { tb_Stress1131, tb_Stress1132, tb_Stress1133, },
                new [] { tb_Stress1141, tb_Stress1142, tb_Stress1143, },

                new [] { tb_Stress1211, tb_Stress1212, tb_Stress1213, },
                new [] { tb_Stress1221, tb_Stress1222, tb_Stress1223, },
                new [] { tb_Stress1231, tb_Stress1232, tb_Stress1233, },
                new [] { tb_Stress1241, tb_Stress1242, tb_Stress1243, },
            };

            if (TextBox1.Text == "" || TextBox2.Text == "")
            {
                MessageBox.Show("Some textboxes are empty!", "Error", 
                           MessageBoxButtons.OK, MessageBoxIcon.Error);
            }
            else
            {
                double var1;
                double var2;
                double var3;

                var1 = Convert.ToDouble
                       (TextBox1.Text, CultureInfo.InvariantCulture);
                var2 = Convert.ToDouble
                       (TextBox2.Text, CultureInfo.InvariantCulture);

                if (TextBox3.Text == "")
                {
                    var3 = (-0.007 * var1 * 2) + 1.8;
                }

                else
                {
                    var3 = Convert.ToDouble(TextBox3.Text == "", 
                           CultureInfo.InvariantCulture);
                }

                var tadd = ((1.25 * var3) - var3) / 6;
                
                for(int j = 1; j <= 12; j++) 
                { 
                    if(j > 1)
                    {
                        var3 = var3 + (((Math.Abs(7 - j) * (-1)) + 6) * tadd);
                    }

                    for (int WP = 0; WP <= 3; WP++)
                    {
                        var S1 = Math.Pow((var2 - var1) * WP, 2);
                        var S2 = Math.Pow(4 + var3, 2);
                        var S3 = Math.Pow(var1, 2);

                        double[] outputValues = new[]
                        {
                            S1 * var2 / 3,
                            S1 * S3 / WP,
                            var3 + var1 * S2 / WP,
                        };

                        for (int i = 0; i < outputValues.Length; i++)
                        {
                            textBoxOutputGroups[WP][i].Text =
outputValues[i].ToString(CultureInfo.CreateSpecificCulture("en-GB"));
                        }                        
                    }
                }
           }
      }

One above only filled the textboxes from the original textBoxOutputGroups leaving the rest empty.

How should I do this?