Seller Financing Structure: % of Purch Px or Total Uses

Question for folks who've used seller financing as part of your transaction:

Do you typically ask for financing as a % of the purchase price or a percent of total uses? Any strong reason to favor one over the other?

(Want to avoid situation where seller potentially balks at prospect of financing your deal fees, cash to the B/S, as part of their 10-20% note. But if it's reasonably standard to have them finance total uses instead of just purchase price, would be helpful to know).