I found the original fitting instructions filed away (in a sealed astronomy box of old gear - in a mostly dis-used outside toilet
).
So I am a bit of a hoarder it seems!
So the instructions on the first two pages should have said rotate the dome so each bracket will be positioned to the exact same vertical height at your desired motor location - rather than just move along the dome when positioning brackets using the three wooden L shaped spacers; that one change in instructions could have saved so much pain!