Each day of the reserve block is given a notional credit for CAP calculation purposes during roster building. Your actual duties will attract the correct credit for the trip or standby duty and the company cannot exceed a total credit for that reserve block without paying you NCP. The figures vary slightly between SH and LH.
In other words, you don’t need to ‘make up’ work if you have a quiet reserve. You’ll just receive slightly less in flying pay and allowances as you won’t have flown much but your basic pay will be unaffected.