Correct length is 375 mm eye to eye centres from factory
Yes you are absolutely correct 375mm is exactly right!
I have found on average a replacement after market hover around 380mm eye to eye, so like you said do the maths get the shaft travel length, most important, as most shockers (not all) these days are length adjustable (eye to eye)
we are setting up suspension atm on a pre75 motocross sidecar have 80mm shaft travel and can have eye to eye 335mm to 350mm
so will get 2 pieces of tube that fit inside (telescopics) make them say 350mm eye to eye long, mark 80mm travel then can position get my shocker mounts and 4" of axle travel
I have also made up a board with a pivot point, swing arm ( just thin flat bar) then can mark shocker positions on swing arm and top shocker mount then give the shocker travel and get the axle travel ( it comes in handy)
cheers