The Revi gunsights are no doubt not calibrated for spitfires. More likely it uses the mil system, marking the rings accordingly. Biggest ring is probably 100 meters across at 1000 meters, and 10 meters across at 100 meters. The ones inside I haven't checked but one can visually figure it out by checking their ratio compared to the main ring. I'm sure the gunnery manual we found in german and finnish tells it correctly.
I found all kinds of pilot manuals for the Bf 110 series (and Me-210's, Me-410's, 109's, Fw-190's, Ju 87's, Ju 88's, Ju-188's, He-111's, He-162's, Me-262's, Do-17's), from early to late models. Even several different ones for various G-2 models depending on loadout options. Including the Mk 108 version.. It even has illustrations/charts showing the ballistic path of the 108 shells compared to the 151/20 shells. It's all in German which I don't understand well. Going to select the most interesting parts and type them into google translate I guess so I can make sense of them. I do look forward to learning how the real pilots flew those things - making use of all the instrument panels for RPM, manifold pressure and so forth.
I found the manuals here:
http://deutscheluftwaffe.de/archiv/D.../Dokumente.htm - Click on "F" and then
scroll down to "Flugzeug Handbücher". Then on the manufacturer.
EDIT: Crappy, been looking through the 110 ones, and they are mainly just technical manuals, not really for piloting. Not the gold mine I thought it was.