If you're going to partner with the seller you want to have everything out on the table. The last poster is correct though, these deals are the wild west.
It's not uncommon for partnership and JV agreements to spell out exactly what each party is expected to do. I've negotiated jv agreements down to who does what and when, like Party A will pay for all development costs, party B will pay for all operating costs after construction is complete and certificate of occupancy is issued, party c will contribute the value of land as imputed capital (as determined by an appraisal) and zero cash.
You'll do the same for returns. For instance, you might specify that after all debt is repaid, all parties will receive their initial investment plus a 10% return, after which time party a will receive 60% of the cash flow, party B will receive 30% of the cash flow, and party c will receive 10% of the cash flow.
There are a million iterations you can go through. The point is to make everyone happy.
Find out what kind of return the seller/owner is looking for, meaning what would make them happy when this project is complete. Would they like to have made a certain amount of money at the completion or a certain % return on their overall investment.
From there, you'll work backwards. You should already have from your comps or market study estimates of what the property will return to you on an ongoing basis (if rental), or what it would return in a lump sum at sale.
After debt is repaid you can figure your net profits to the overall LLC. If that number is enough to give your seller/partners what they're looking for and also give you a return you can be happy with then you are set. Put it all in writing. consider using an analyst to make sure all the numbers work out and an attorney to make it all legal.
Call me for help. I enjoy working on these.
Good luck with it.