I have installed many scopes for my own use - some will disagree with this: Imagine there is a line down centre of your barrel's bore - you want to install your scope so that the vertical cross hair would pass down through that imaginary line - so you want your scope centered and "square" to that barrel. If the cross hairs are canted - tilted - then as you elevate for range, you will be pointing the bore further and further off to one side or other.
In practical terms - I try to set the vertical cross hair to seem as if would pass down through centre of firing pin or cocking piece on rear of the bolt - as you move your eye out of the "eye box", will tend to see ring of black - so keep the bright clear centered - to keep sight of that cross hair, and keep it in the middle. I have luxury of various cradles and padded vise to hold rifle when I install a scope - so I can let go of rifle to fuss with scope to get it lined up. I had read that some just take cardboard box and cut "V" in each end - balance their rifle in that - to get their scope "straight". For some number of years I have been using an inch-pounds torque wrench - I look up specs on Internet to find what inch-pounds does maker call for - I do not use "rule of thumb" or one number for any brand / installation - look up what the maker calls for. That goes for screws into the receiver to hold scope base, cross clamp screw and cap screws. I do put one drop of BLUE thread locker into cleaned out threads in receiver to anchor the scope bases - I have NEVER used any form of thread locker on the cross screw or the cap screws.
I once owned a Schultz and Larson "free rifle" target .22 gun - set up for 50 metre bullseye - front and rear aperture sights were significantly to one side of barrel - so it can work - have taller rear sight to accommodate range (trajectory) and left to right convergence to have Point of Aim equal Point of Impact. However, double the range, now will have Point of Impact as far over as the rear sight is - triple or quadruple the range and Point of Impact keeps going further and further from Point of Aim - because sight is not "in line" with the bore. Will be same with you line of sight through scope - if you have installed cross hairs to be canted, and then you cant the rifle when you aim - you will get cross hair aimed at one point - and at one range is possible to get Point of Impact there - but at any other distance - closer or further - the Point of Impact will not be at the Point of Aim.
