Here is how you can calculate a lease payment.

(MSRP - residual) divided by # of months in lease = deprec portion of payment
((MSRP + (plus) Selling price) - (less any money down))x money factor = money portion of payment

add the deprec portion + money portion and you have your monthly lease payment, plus tax(es).

If you want to add your old car payoff to the new lease, you need to add that amount before you multiply the money factor.

In a lease, the residual and money factor can be used by leasing companies to get you a great lease price...usually by inflating the residual, which kills you if you try to end the lease early. Right now, I would guess the money factors are about 0.0025. which if you multiply by 20 gets you the equivalent rate of 5%...