hi mate
nice choice i use the same cam and 70-80m won't be a prob
first up you need to us the cam on a parallax adjustable scope, ideally side p/a just easier to use.
the distance between cam and scope not crucial but i have mine quite close.
when all fixed on set the cam on to manual focus then focus the cam on to the crosshairs so they are nice and sharp then you just leave the cam alone well may be just zoom in a little if needed to fill the screen on the cam.
(you will need to set to manual focus each time you go out as the cam resets to auto focus after 24 hours.)
then you just use