I guess you can set just the bare distributor in and turn rotor till the drive tang drops into the pump slot and mark or measure to a place that won't be covered by the clamp assembly. Remove and reinstall with clamp and plate in place and see if it drops down to your mark or measurement. I was thinking the housing had a step that stopped the distributor at the top of the clamp to give correct spacing but it's been many years for the old brain to gather cobwebs.

Evan