I had something similar and it was because the denominator was a zero before the user entered the variables. It was fixed with an "if/then/endif".
It looks like the problem in yours is form1 being zero. Try
if([YOUR DENOMINATOR]>0)then[YOUR FORMULA HERE]endif
if(form1>0)then(this.rawValue=form1.#subform.NumericField4.rawValue / form1.#subform.NumericField5.rawValue)endif
Your formula is a bit more complicated than I can decipher, so I'm not sure how much after the "/" you need to include to capture the part of your denominator that is making it null. You may need to insert everything after the "/" in the ">0" parameter. The goal is to tell it only to execute the calculation when the denominator is greater than 0.