What is JotForm?
JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
At JotForm, we want to make sure that you’re getting the online form builder help that you need. Our friendly customer support team is available 24/7.
We believe that if one user has a question, there could be more users who may have the same question. This is why many of our support forum threads are public and available to be searched and viewed. If you’d like help immediately, feel free to search for a similar question, or submit your question or concern.
For what I understand, you are not having difficulties withe the calculations, you just don't want to receive a submission with a negative results?
If my assumption is correct, you can simply create a condition to show the submit button only if there are positive areas, like this:
Please try my example here: http://form.jotformpro.com/form/50646548344965?
Let us know if you need anything else, we will be glad to assist you.