My experience parallel's Marco's: I can focus more accurately [and far more quickly] without a grain focuser. It helps that I am terminally myopic. If you are far sighted I am sure a focuser is a great help.
The magnifiers' adjustment isn't for calibration but for adjusting to your eyesight so as to bring the reticle into focus. But you aren't making a print of the reticle. The proper calibration is to either move the reticle or to shim the focuser.
The Ctein article on focusing was discredited long ago.