In filemaker, there's no way to create a field dynamically, u have to create a field explicitly. And for the calculation, u need to goto manage->database, then autocalculation for each calc1, calc2,... and in "calculation" u can specify calculation ( if(answer1a != "") and if(answer1b!="");answer1a - answer1b;"" ), so u don't need to calculate the value answer1a - answer1b, and so on.
Ok, I understand the first part of your answer, but not the second.
Where are you putting the calc? What I have done would be less typing unless this calc goes in a single place, or can be pasted into each calc field.
I think you should restructure your database so that you don't have such a large list of dedicated fields. Put each survery response in a separate related record. To add a new survey response, just add a new record.
If you search this forum using Survey as a keyword, you'll likely find a number of threads discussing surveys managed via FileMaker that go into this concept in much greater detail.