If Live View autofocus is accurate but viewfinder autofocus is not, that usually points to a phase-detect AF calibration issue rather than a general focusing problem. Since you noticed it mainly with the 50mm f/1.8 II, the lens could also be part of the problem.

The safest advice is professional calibration of the camera and possibly the lens. Adjusting anything behind the mirror yourself is risky and can make alignment worse. Consumer software adjustment is generally not the solution here, and the EOS 450D does not offer user AF microadjustment in-camera.

So: don’t try to turn internal screws unless you’re prepared to risk misalignment. If possible, test with other lenses to confirm whether the fault follows the camera or only that 50mm lens, then have the affected item calibrated by service.