I am not sure what fair is. We leased a few Trojan cows 30 years ago. It was a 50/50 split but we got the first pick AND chose the sires. Why not just pay them to take care of them if they are not making any genetic decisions? Maybe they want those genetics already as we did? Anyway, I think it is a great way for an old rancher wanting to hold on to cattle and help a young rancher get started without the upfront cost of purchasing the cattle.
You need to decide on several things before you look at split on the calves.
First do you want cows at the end of the contract. If you do who is replacing culls and mortality. Also who supplies bulls.
Second part is management risk. What are the target for calf crop percent. For example any thing under 92% comes from operator share and any thing over 97% goes to operator. Also vet bills and breeding cost if not bulls.
The few I've seen are about 80% to operator with the herd owner getting 20% and the herd owner looks after replacement and gets culls. This would assume you want a herd after the contract is over. They were for short-term and I would expect some changes for a longer term.
You also need to think about keeper hiefers as to split. They are generally done separate from the rest of the calves that are percent of receipts.
Open it and plug in all your variables for your situation.
I really like this way because it's so hard to factor in everything that goes into making the lease fair, and this way you have some objective calculations to support your negotiations. Hope everything turns out well for you!