Calculations are not supported for Formscentral HTML forms. PDF forms created in Acrobat do support calculations. Depending on the type of workflow you desire it is possible to import PDF forms into Formscentral and collect your data there.
It is currently the 2nd most popular idea/suggestion that has been made behind multiple fields on one line which has already been delivered.
Is it even being considered? There are other companies offering this already in their software & this really should be in Adobe's sights.