Minimum front sight insert size should be at least 2 times the size of the aiming mark. Looking through the front sight and moving it to the edge of the aiming mark (gong) it should not take up more than 1/2 of the insert. The bigger the insert the better. If you find that a 3.4 insert looks good, go up at least one size to a 3.6 or 3.8. If I was coaching a new shooter to shoot iron sight from 300 to 1000 yards I would glue a 4.0 into their front sight and tell them that is the only size there is. The more white there is around the aiming mark the easier it is to center.