input string was not in a correct format - Why am I getting this?

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C#

While coding some libraries I ran into some errors with my string format.
I don't really know how to solve this, or anything about string formatting.

My Code:

public float getMeleeCB(float attk, float str, float def, float hp, float pray)
        {
            return (Convert.ToSingle((double)Convert.ToDecimal("0.3f") * (1.3 * (attk + str) + def + hp + (.5 * pray))));
            //example of use: string test = "Melee Combat Level: "+getMeleeCB(60,53,58,53,47).ToString();
        }

Run-Time Error Message:
Format-Exception was unhandled - input string was not in a correct format


How do I format it to work the way I set it up?