Being able to turn off pro-rata calculations for clients

My client doesn't like the pro-rata calculations so likes starters and leavers salary to be adjusted manually. This client has over a hundred employees and frequent starters and leavers each month, so once I've done the manual adjustments and calculated the payroll, if I have to undo the calculations to make amendments to any employees this then losing the manual adjustments on starters and leavers. Therefore this is causing extra work because of having to check and manually adjusting the starters and leavers again.