I did some work on this a while back - a B engine will turn 2.5 rpm for every rpm the pto turns. The Owner's manual says that a pto rpm of 545 is reached at an engine speed of 1362 rpm. So, if you're trying for that 1500 rpm rated engine speed the pto speed should be 600 rpm.
McMaster-Carr has a nice hand-held tach you can buy for around $65. Not a bad investment if you need to set engine speed, idle speed, and/or check pto speeds on various tractors. You'll be suprised at how much some tractors' indicated pto speed (the mark on the engine tach) is off. Our 190XT with the hydraulic pto clutch was off a fair amount (slower than indicated).