I've been a co-founder of three ventures and have had to negotiate ownership stakes with my partners for each one. I've also conducted intellectual property diligence, as a consultant to IP/M&A attorneys, for multiple multi-million dollar software M&A deals.

As Stoney said, it's a bit hard to come up with a firm number, given the number of unknown variables involved here. Also, it almost sounds like you and your product are being acquired by this larger company, while essentially selling or licensing a product you've built to this company in the process. If that's the case, you may want to speak with an attorney who's experienced in software licensing agreements and M&A. They may be able to steer you in the right direction of a valuation, as well as make sure you're very clear in what you're giving away, from an intellectual property rights perspective.

Another thought that comes to mind is that if the company is providing you with a lot of support, including a salary, be prepared for them to possibly low-ball you on the ownership stake. Once you decide on an ownership stake to suggest, I wouldn't be afraid to start at the high end of your estimate, taking into account the fact that they may try and negotiate the share down quite a bit, given the financial contributions they'll be making. In my experience, investors putting money on the table typically and unfortunately don't value sweat equity too highly or fairly, when compared to cold, hard cash.

Always happy to discuss further on a call and good luck!