@jj12stars - in line with my colleague posting a commercial solution, I would suggest that this kind of customization is well within the scope of form enhancements. Basically, the approach is to overload the form embed script (usually within a custom landing page module) with Javascript that performs necessary form manipulations and calculations. The final ‘submit’ operation combines all the entered values and derived values and passes them to the related HubSpot object.
A very brief reference on this (for legacy forms at least) is at: Legacy forms - HubSpot docs I’ve done more work using these features than I ever would have expected!
Hope this is helpful.
Steve