Calculate the circumference that the blade tips cover in feet the mulitply that by the rpms
So a 20" mower has a circumference of 5.25 feet x 3000 rpm = 15,750 ft/min
Or if you divide the max speed by the circumference you get the max allowble speed
19,000 / 5.25 = 3620 rpm max speed.
As there will always be some over reving on governed you would set that engine to 3400 rpm allowing +/- 200 rpm.