I'm not sure why you're using a biz form for your calculator. Your "calculator" should be custom code, then if you want them to sign up or submit their info before or after using the calculator, you have that be a biz form independent of the calculator. Don't try to make them do one in the same as they are different activities.
If your calculator is posting the page back and submitting the biz form, then wrap the calculator in an update panel and this should resolve the issue.