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.
I'm Autistic, if I say something you find offensive, please let me know, I can guarantee it was unintentional.
I'll try to give my honest opinion but am always open to learning.
Mark