Hi Bob,
The shaft merely has to only protrude in by about 3 mm. To achieve this you should have between 3 to 5mm of gap between the back of the clutch body and the oil seal / PTO surface exit of the engine.
Once you've achieve this the clutch fork should be adjusted so that it's pretty much perfectly vertical (perpendicular to the engine deck) when the drive is in the engaged position and leaning slightly towards the engine when the clutch has the drive disengaged.
Cheers,
BB.
