If you walk away right now, you own 40% of the company. There are some business-world dirty tricks Jim can do to cut you out without paying, but they can take some time to pull off, and some are grounds for a civil lawsuit (in the US).
That 85:15 ratio in your favor should have accumulated some sweat equity. The initial cash infusion works out to 7.7% you, 92.3% Jim. In order for that to work out to 40% you, 60% Jim now, the company valuation with minimum-viable product should now be $22333.33, and the value of the work you added via labor should be $7933.33, and the value Jim added by labor should be $1400.
If that seems reasonable, so too should the 40/60 split.
The value produced by the programmer-contractors doesn't count toward equity. They converted cash into company assets as a consequence of the business structure you set up. The only reasonable ways for Jim to ethically increase ownership share after you leave is by putting in more sweat equity, or by infusing more cash directly. But legally, the ownership share was established with the expectation that Jim put in more cash, and you put in more work, and anything happening after would require renegotiating the agreement.
With respect to Jim contracting the developers directly, that would not matter in the US (with a decent lawyer). Clearly, he was doing that as an owner/officer of the company, so the work product belongs to the company. In business shorthand, he loaned the cash to the company he owns, and then immediately paid it out as majority owner to a contractor. The work product goes on the books as belonging to the company, along with a zero-interest debt to Jim. Or perhaps the initial capitalization was in the form of IOUs from Jim to the company, and in paying the contractors, he simultaneously redeems those IOUs. We can't say for certain without seeing the incorporation documents.
You are actually being too reasonable. Demand an independent valuation of the company. Take 40% of that as cash buyout. If the company valuation grows over time, take 40% of that, whenever Jim feels like buying you out. Jim is trying to lowball the current value of the company, in order to screw you out of the value you put in after the initial investment.
Remember that whatever deal Jim may propose to you, you could use the same valuation strategy to buy him out. If you could be bought out by paying your share of the incorporation fee, would it be fair if you paid him his share to buy him out?