Heres the Saturn pic broken down into 10mm incriments.
Comes in around 225mm to the back of the jig so even more confusing. Ill let Phil cary on with what he is doing and get on with some work now...
edit: OK optical illusion. Looks like the 200mm is measured from the back of the jig on your pic Johno, it must be to the back of the bush tube and the line carries on. Hard to see in low resolution. Changes the track at the back somewhat.
Cheers
Stot