I'm with wogpotter - invest in a sight drifter (if your rifle is the sort to need one). It should be possible to find them online - I bought one a while ago for SMLEs, which came as a parcel with a bunch of other useful armourers' tools. You can do the adjustments a lot more precisely without the hit-and-miss of hammer and drifter. I've tried that way, and I'm still not convinced I didn't knock something out of true.