The way I am reading the OPs post, the landowner is footing all the bills - purchase, feed, equipment, vet, dead cart, etc., - and the OP is paying for nothing but doing all the work. Any income is then split 50/50. Which seems like a reasonable exchange to me, at least while they feel their way into how this all works out for them.
So my cautions would be only 3, really :
1. Be clear about ancillary spend too, eg., transport, sales fees, luck money, butchery. Just so you all know where you are.
2. Check what approval the vet might need, as it is not the person looking after the sheep who will pay the bill. They won't want there to be any ambiguity (and you won't want there to be any delays) about whether you can authorise, for instance, a caesarian that may cost more than the value of the ewe or lambs. (Or even a site visit, some vets may be reluctant to visit if the caller is not the owner or an employee.)
3. In your agreement, whether it be written or not, have it very clear that either party can request a change in the arrangements after the first year, and at given points after that. And the partnership to be dissolved amicably if no amended agreement can be reached - preferably on a basis agreed now, eg., sell the sheep and split ... the proceeds? or the profits? If the latter, what costs are included in the calcs? That way, hopefully no-one ever gets disgruntled and feels taken for granted, because if the deal turns out to be inequitable - in either direction - then it can be discussed and amended with no ill feeling.