What speed is the tiller designed for?
I think there are two "main" designs, 540 RPM and 1,000 RPM. For my 540 RPM I need to run my M4700 engine around 2,200 RPM.
Do you have something on your dash that says PTO speed?
How many teeth on the (tiller) PTO spline?
6 usually means 540; 21 usually means more.
If it works well at 540 PTO RPM with engine at rated PTO RPMs (look for a mark on the tach/check user manual) and low ground speed (< or = to about 1 mph) then that's a good speed to run it.