I am also thinking about a Tremmory for my JTV-69...and also am unsure about which length unit to buy. I guess that you would have to screw the claw plate as far as it will go towards the front of the cavity (as above ) and subtract 3-5mm from the distance between it and the front edge of the trem block.
I was wondering if they mean 3.5mm rather than between 3-5mm?
I also read somewhere on another forum that you can replace the thumb wheel with an Allen head screw (not sure whether this is supplied)
Be glad of any other info...