As mentioned in the comments, you need to check for zero first. In the what I have tried section you wrote code that if SqIn is 0 then divide by 0. The statement is redundant and does nothing.
A trick you might be able to use is something like:
combo / (SqIn + 0.0000000001)
What this does is makes it so that divide by zero most likely won't ever happen and if you don't need great accuracy in the math, this is a workaround.
Or, just check for 0. Simple.