Measure your height above bore and use a ballistic calculator to determine where your setup first crosses LOS (zero). I use this method to boresight a new scope by looking down the bore at a target and aligning the scope with it, generally 34 to 38 yards away. Always on paper at 100 yards which is the goal. I might shoot one or two shots at home to confirm before heading to the range just to confirm.

Life Member NRA, RMEF, American Legion, MAGA. Not necessarily in that order.