It all depends on what your goals are and what the goals of your partner is. If you both want the same thing, WRITE IT DOWN. Don't take anything for granted. Partnerships only work when each partner knows his/her role. There are many ways to lease cows. I would clarify the following:
How many cows and for how long.
What the percentage of calf income each party gets.
Who pays for all incidentals (registrations, vaccinations, treating sick ones)
What to do on death loss, who is ultimately responsible regardless of a dead calf backwards or a cow that fell on the ice and broke her back, or neglegence on either party.
Breeding/Flushing decisions
Who get the eggs/pregnancies/splits on calves.
If, the lease is terminated, how is it handled to compensate both parties.
buy back options on calves, cows, bulls, embryos, pregnancies.
It is amazing how many freindships have been lost over leases on animals. I won't go into the bad stories, as there are plenty. Just make sure you both have a clear understanding of what is the expectation of each person involved PRIOR to the endeavor. If you need more, I am sure others will chime in.