With a magnetic muzzle collimator, I place them as close to center horizontally as possible on the muzzle, but also as close to the same height above the bore as the scope.
Don't really understand what's the problem is with getting them centered horizontally. This is easily done by eyeballing.
As for "precisely vertical," I've already mounted the scope so it's as square as possible with the center of the action/barrel, so I just align the collimator's reticle with the scope's reticle.
Are you looking for some system involving spirit levels, or something else to check your placement?
