I’d reiterate @CWOTUS‘s point regarding this technically being part of your job, not because it necessarily means you should get less, but just because it absolutely means you should get your deal in writing. At any time, if things go wrong, without something in writing, you probably don’t hold anything because you’ve been paid for your time and they are putting up the investment. Again, not trying to minimize your contribution, just sharing my understanding of this sort of set up in a legal sense.
With that over, the simplest thing would be that everyone places a general value on their work, expertise, or financial contribution, and gets a piece of the pie that roughly matches that as part of the whole. That’s complicated a bit with your normal salary, but might be a good starting point.
You might also want to share in some of the risk for a larger possible reward. You don’t always have to, but it puts everyone in the same boat and ideally would get you a larger piece of the overall partnership.
Also, profits will be separate from ownership usually and the two aren’t always connected, so make sure and discuss how you would actually get a check. If you’re thinking they’ll take profits out, and they’re thinking everything will be reinvested that can be a problem down the line if you’re waiting on a check. Make sure you’re on the same page.
A real life example: Right now I’m working on marketing a product with someone I’ve worked with before on a contract basis. He’s putting up all the funds for development and he needed branding, packaging, web work, and online marketing. I placed a value on the work he needed, took a small payment up front to kick things off and commit us both to it, and then take a small payment for each unit sold up until a bit more than the value of the initial work. I’m also getting a piece of the overall company which we’ve assigned an approximate initial value to.
I’m sharing in a bit of risk with the staggered payments that only come after sales, so I get the ownership stake to offset that.
One last piece of advice, humbly learned. Tons of people told me for years to never offer a number first in any negotiation. Maybe that works for some folks, and maybe it’s just personality based, but it didn’t work for me. I’ve started instead just offering a number that would make me absolutely want to do the deal and I’m surprised how often the first offer is taken and we can move forward, both excited about the project. If there’s a number that makes you want to wake up in the morning and tackle the thing, and you can back it up with good reasoning, I say throw it out and see what they say.
I hope it works out for you and good luck.