Wide open. But if you get over some dirt or dusty areas don't be afraid to back off the throttle, then throttle back up again when you are back on grass.
That is what you are supposed to do. Never, ever, ever start a PTO at WOT. It's like doing a clutch dump. Start off slow, then move to full power once everything is up to speed. Plus it is more forgiving if something goes amiss.
Kubota M8540HDC, L5740HSTC, BX2200, BX2660, John Deere 425&1025R, Case, Massey Ferguson, Ford
According to my manual the MMM and rear PTO have slightly different 540 ranges, one is 2900 and the other is some odd number like 3025 or something like that. There is so little difference between the 540 and WOT, to make no real difference. The manual addresses RPM during break-in, but not much after that. My 2003 BX2200 had no tach, so I just pegged it and its still running great.