I think the formula would be something like Purchase price + Insurance + RAC membership + Estimated servicing/tyre costs + Admin Costs + Dealership payments + profit – Estimated sale value = Advance Payment Required. Even then there’s too many variables to be able to predict anything other than the AP will be what Motability say it will be and what the manufacturers feel they can give in terms of discounts will help Motability reduce that payment.
The profit is an interesting one as in theory they’re not supposed to make one.