I found this on the net by chance...AND IT WORKS!!!! Even though it is lo-tech!
Take your scope off the mounts.
Take 2 small planks and cut an equal vee in each. Place these on another plank and use prestik to keep it all in place.
Align the planks and adjust the width apart so that your scope is supported along the tube at its furthest points, and as it would be on the rifle (turret position)
Place a stable level surface (a table) up against a vertical wall mirror (the mirror must be as close to 100% vertical as possible.)
Place the scope setup facing the mirror, about 15-20cm away. Make sure that the scope is facing the mirror at right angles L-R and Up-Dn (Prestik and sheets of paper can be used to do the minute adjustments or just get a straight edge (set square) and place it against the mirror, and parallel to the scope for better (and easier) visual reference.
Now look through the scope.
You will see the reflected reticle.
Adjust the vertical and horizontal reticle position until the real and reflected reticles are exactly aligned.
Aim the plank setup at a long distance object and revolve the scope in the vees while looking through it.
Check how much 'wobble' you get (if any). If the amount of centering is not acceptable, repeat the process.
Although this is not the most accurate method, I did it in less than 20 minutes after the cutting was done.
I took my scope, bolted it onto the rifle, and was only 100mm down and 20mm right - first shot at 25m. I then had PLENTY of 'clicks' to play with, while zeroing.
Hope this helps.