Thank you for answering my questions!
The herders/cattle of course a super productive, no doubt - but they would be even if you left them to graze somewhere else or on the fields directly. So I wouldn’t count the fields they’re grazing on as “fully employed”, as they don’t get you any resources that you wouldn’t get if your cows were grazing somewhere else, be that an empty area or by feeding on your crops (while you would still get their fertilization effect in the latter case).
I guess if staggered rotations and apiaries don’t keep your farmers near their fields (and setting them to labourers and back to farmers mid-season is too micro-intensive), having them contiously re-till old fields, essentially, does the trick.
I never needed full farmers for the bigger fields when we’re just talking planting and harvesting. Not even when some of them have wandered off in between. The only time they don’t get the harvest done completely is when they bug out and leave a tile or two unharvested for no absolutely reason, leaving it to rot. But that doesn’t change with more farmers assigned.
25 farmers should be enough for 3 fields of 286 tiles. Somebody correct me if I misremember.
I haven’t run a big test like this, but I don’t think clover is the most efficient choice. Imho, leek+carrot nets you the highest yield, followed by leek+buckwheat. Both are -6% fertility though. Peas and leek is only slightly less, however, and only -3% fertility. I wonder how big the gain from cows munching on those is - because even two herds will only cover 10x20 tiles out of 12x24, it should be lower than your numbers. I haven’t used them in this way yet, so I should probably test it. You got +3.33% on average per year of grazing, which would be enough to contiously run peas+leek without any compost at all. Not sure about fodder quality though.
Even without the cows grazing on you active fields in exchange for some of their yield, you would only need 3 loads of compost over 10 years to counter the yearly -3% from peas+leeks. And with fields three times the size of your 10x10 fields, you could afford that. I think that is the most efficient setup, though of course planting peas+leek every year on all three fields probably means a constant loss of 10% or so of your yield to diseases (still worth it) and no flax, grain etc.
Throwing in years with buckwheat+leek costs you -6% fertility. I don’t know how well flax+flax or flax+beans really works, I haven’t really measured how much you lose to frost spells with those (especially the latter). Add one of those to the 3-year-rotation and you lose -12% (with double flax) or -10% (with beans) over those 3 years, which compost got covered. No clover needed. Less disease loss too with a double flax breaking up the mildew crops. And thanks to having three fields, you’ll get a continous supply of all of those crops. Might not be enough winter fodder for the cattle though only 286*11 = 3146 units of grain per year for both your millers/bakers and cows. That’s average yield before any fertility bonus, though. Leek+carrots would offer about 600 units more.
I’m really looking forward to your results vs the ones from Beetrix, though I don’t really know their setup.