calculating percentages based on checkboxes

In that instance if they are numeric fields I would suggest using <>0, which is the formula notation for is not equal to 0. Then it will check the numeric field is not zero and run the math. This will try to run the math if that number in those field is ever negative though as well. Where something like [Field B]>0 will only do the math if that field is a positive number, depending on how the math in that field works out.