</font><font color="blue" class="small">( What gear should I be cutting in? )</font>
FWIW, your manual should also state that mowing at LESS than 540 rpms is not recommended either. The gear selected for propelling the tractor has nothing to do with producing PTO speed. That's strictly a function of the PTO speed select lever (if you have one) and the throttle. With the PTO spinning the mower ~540, you should then select the transmission gear ratio that produces the best cut behind the mower.
You put the speed select lever in 540, and adjust the engine rpms accordingly. I know that's difficult without a tach, so you'll have to use your ears. PTO rpms are generally about 80-90% of full throttle. Advance the throttle to full, then back off enough to hear a reduction in the rpms. That should get you reasonably close to 540 PTO output speed.
//greg//