The Theoretical Amount In Budget Module refers the Budget Amount (Planned Amount) which should have utilized theoretically on the basis of the Days elapsed in the duration suggested for the Budget (i.e. Start Date to End Date of the budget).
There are two scenario of calculation of Theoretical Amount:
1/. If you have specified a Paid Date for the Budget i.e. the date on which Budget Allocated, is greater then the End Date of Budget then it will show you Theoretical Amount = 'Zero' as Budget Amount should have been utilized.
2/. It also check with the current date if the days remaining from current date to End Date of the Budget. It calculates the Theoretical Amount via Following Formula:
elapsed_days = Days elapsed till current date
total_days = Duration of budget in Days
planned_amt = Amount Allocated for Budget
Theoretical Amount = (elapsed_days / total_days)* planned_amt