Mr Azuaro is correct In this case, adjustable mounts can take you to a world of hurt. If you do what he says your rifle will be set up correctly, if you simply just buy a set of adjustable mounts your possibly just masking a problem that exists already. If you measure a set of sportsmatch mounts or even just slide them up to each other on your rail, you may well look for another brand or lapping tools. I'm not knocking sportsmatch as they are in essence a more budget orientated mount but if you want your rifle scope combo to run straight from breech to target then AZ's post is the answer.
To answer your next question which you will ask Monday morning, crossover is where the scope is not directly above the barrel.