Bro, you'd have to be a robot to hold it still.
You place the front bell flush against the glass with a bright light overhead of mirror. A bathroom make up mirror is best for this.
I check the reticle at lowest power, highest power, and whatever I prefer to shoot on, which is usually 6X if variable. Though irrelavent I know mils are true at 10X or highest power of scope, some at whatever given magnification manufacture states. I always put a yard stick at 100 yards or meters, depending what scope is calibrated for and check the reliability of the mils calibration.
Moving on. You'll find that counting total rotational travel of clicks & halving it will never be the same as optically centering it. One of the important things to consider when centering your mirrored image of reticle is try to place your eye inline with ocular bell same as you would if on action in stock.