Where should the needle be when running at 540rpm ? When I have it at 540 ( the line ) and I bush hog it drops down a little. Should it be at the line/star or anywhere in the brackets ?
On the line is the "right" place, but I sure wouldn't worry about it anywhere in that bracket. It's certainly not unusual for RPM to flluctuate a little bit when brush hogging and going from a light load to heavy load and back.
The main problem is that if you overspeed your implement you might start leaking oil past seals in the gearbox and then pretty soon you'd be looking at some repair costs. I doubt if you could run things fast enough that there would be a serious concern about the blades breaking or something equally disastrous.