J range Chain tension setting

[57Speedster]

Me again, in need of some advice. I'm trying to find out how to set the upper chain guide on my Cento engine.
The only instructions I can find on setting the chain guides, seem to be for other Lambretta Models. Maybe I'm just thick, but I'm just not understanding how it is done on a J range chain.
The Instructions read : Turn the clutch until chain is tight on the lower run, then move the upper chain guide so that there is half an inch (12.7 mm) up and down movement at the centre of the top chain run, then tighten the bolts on the upper chain guide plate.
The problem with the Cento and other J range upper chain guide, is that they press the top run of the chain downwards, (I understand other models push the chain up), and when in position, the J range upper guide covers the chain, so you can't see or measure the movement in the centre of the top run.
I've fitted a new chain and both sprockets, so there isn't much slack anyway, and I'm concerned about causing excessive wear to the chain and/or chain guide if I set it too tight.
Any advice much appreciated.
Cheers
57Speedster