That all depends on the number of notches cut into the reverse lever quadrant in the cab. I know of a couple of engines with 50 to 75 fine teeth in their quadrants, and a like number of positions to set the valve gear, and I know of one with only 15 or 20 fairly coarse teeth, which only allow a fairly coarse adjustment.
With a fine tooth quadrant, and the power reverse in good condition, it is easy to run with the reverse, with the throttle wide open. With the coarse tooth quadrant, it is almost impossible to do so smoothly, except with a very heavy train.