COMBINED COVERAGE AMOUNT FOR MULTIPE PLANS IN DIFFERENT PLAN TYPES IN OAB

I have a case where a combined coverage amount for a couple of plans should not exceed a defined limit. One has a priority over other. Can anyone help me figure out how I can automatically calculate the 2nd max amount by subtracting the defined limit  minus the coverage amount on the 1st plan, other than using postelection edit FF? I think post election edit only gives the error message.  This is in R12, not in Fusion.

I could probably use external function, but not sure if the calculated amount will be there in the database for the 1st plan while processing the life event.