So, use the rotor nut to turn the motor. Usually need something to track the piston through the spark plug hole to verify its actual top of travel. The existing mark should be around 5:00 5:30 as in the pic TDC position. Make a T mark on the case. Now we figure out the size of the rotor circumference, translate that to degrees rotation and make an F mark and advance mark on the case. So we need rotor diameter (preferably in mm)