Center to center of the primary and secondary bolts is where you measure from. Clutch offset is measured from the back side (closest to the motor) of both clutches.
Center to center is 268.5mm +/- 1.5mm
offset is 16mm +/- 1.5mm
Easy tool for center to center is a length of wood or steel measured to the correct length then put nails or screws through at those points so they can contact the bolts.
Adjustment is made by loosening and moving the motor.
Offset is primarily adjusted with shims in front and behind the secondary.