You just have to make sure you take Jerry's advice and move your eye side to side lining up the reticle with the left side of the slot and the right side, to make sure that it's perfectly parallel with the slot. Then make sure the pointer is aiming at the very center line of the tang or bolt shroud/firing pin, and you're good to go.
I received my "Blem" yesterday. Looks perfect to me. I've been playing with it on a few scopes that are already mounted. I've found that in addition to Jordan's/Jerry's advice, looking back through the objective with the rifle in a rest/vise provides a smaller site picture that also helps confirm that the reticle is perfectly parallel with the slot. It's a very nice tool!
