For a rear PTO cutter, it depends on what you're cutting. If it's anything tall, or with a thick stem, you'll want to run at, or close to, "rated" RPM. Our field is a former corn/bean field, presently "growing" mostly ag weeds, in the early stage of a prairie reconstruction. I run at "rated" (2700 engine = 540 PTO on a JD790 [5:1 ratio, easy math]) most of the time, just so I can run the machine itself fairly fast on the field. When I want to slow down, rather than selecting a lower gear, I can just back down the hand throttle to about 2200 RPM (= 440 RPM PTO), and the cutter (cheapo 5' KK) doesn't skip a beat. 'Course, I have nothing more than about 3/8" diameter to cut (if that), although it's sometimes several feet tall. If you have thicker/taller/tanglier (is that a word?) weeds to cut, you'll probably want to stay closer to 540.
No experience with an MMM (we don't even have a mid-PTO).
