If you think the rpm from your hyd motor will be around 550, then rpm times 1.92 = 1,056 rpm brush hog shaft speed.
The blades will cone up to speed based on the tractor rpm/pump GPM's fairly quickly. and the speed of the brush hog shaft will be based on how much hyd fluid you put to the motor. Are you going to use a variable speed valve like the one below, or a solenoid valve to actuate the hyd motor? You should have a relief valve across the hyd motor.