When I measured mine with the dti I had the head off with the motor clamped in my vice and the dti attached to a strong magnetic clamp and positioned it such that the action of the gauge was the same angle as the bore. Thus the angle difference between the plug and the bore was eliminated.
With the head still instu, however, I used the method described by Brian.
Another factor I discovered whilst using the locking tool was the bolt holes that it locates into can pick up dirt and clutch dust, particularly if running the clutch in out board configuration. Cleaning it out made the difference between a fail and compliance!
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