Residual is taken as a share of MSRP: residual = MSRP × residual%. Down payment, trade-in and rebates lower the financed amount (the cap cost), which lowers the monthly payment.

adjusted cap cost = selling − down − trade − rebates (+ fees, if rolled in)

monthly depreciation = (adj. cap − residual) ÷ term  ·  monthly rent = (adj. cap + residual) × money factor

By default sales tax is applied to the monthly payment, the most common U.S. method; the toggle in the payment readout instead collects the entire tax at signing. Either way the total tax is identical — only its timing changes, so the total lease cost is the same. Some states instead tax the full price, the cap cost reduction, or the sum of payments — adjust expectations accordingly. Each fee can be rolled into the lease individually (financed across the term) or paid at signing, using its own switch; the master switch flips all of them at once. Out of pocket at signing = down payment + first month's payment + any fees not rolled in + any tax paid up front. Security deposits and the lease-end purchase fee are not modeled.

The deal quality % is computed as (dealMonthly ÷ MSRP) × 100, where dealMonthly = (dep + finance(fee-free cap) + (acq + due-at-signing) ÷ term) × (1 + tax rate). Finance is computed on the bare vehicle cap cost (selling price minus reductions, no fees), so rolled doc/DMV/reg/state fees cannot inflate the money-factor component. Acquisition and due-at-signing are always amortized over the term whether rolled or not. This makes the metric stable across different deal structurings — rolling or not rolling fees, and paying tax monthly or upfront, all produce the same deal quality %. Documentation, DMV, registration and state fees are excluded entirely.