There's no bright-line number, and any state-specific answer here would be wrong somewhere. What courts and state statutes look at is the substance of the arrangement, and the factors that keep coming up are long term, large accumulated credit, the tenant carrying taxes and insurance, the tenant carrying all maintenance, above-market rent, and a strike price fixed far below likely future value. Your deal hits nearly every one of those at once. Several states also have specific installment land contract or lease-option statutes that impose recording, disclosure and cure requirements once certain thresholds are crossed, and a handful treat these arrangements as sales outright. Which statute applies to you depends on the state the property sits in, and that determination needs a real estate attorney there.
On what you can actually change. A 60-month term with 40 percent credited is the aggressive end. Shorter terms of 24 to 36 months and credits in the 10 to 25 percent range look far less like a financing arrangement. Keep the rent at market and price the credit as a genuine concession rather than a premium you collected back. Leave taxes and insurance with the owner. Cap tenant maintenance at a dollar figure. Keep the lease and the option in two separate documents with independent consideration for the option.
The thing that will actually decide your business risk is not the recharacterization question. It's that if a state does treat the deal as a sale, your client may face foreclosure timelines of many months while collecting nothing, and forfeiture of $62,400 of credit is exactly the fact pattern that draws a consumer protection claim. Also confirm whether setting these up for owners for a fee triggers licensing where you operate. That varies by state and the boards do enforce it.