My front sight is pushed all the way up just a hair before touching the front bead.My rear sight is on thesecond section where the rib levels out,just to make sure it is on a level piece of the rib.Here is a pic of my rear sight,and you can see how the rib slopes up from the barrel.The sight is mounted on that2nd flat section.You could put it on the first flat section closer to your face,but it made the rear beads look too large for me,so I moved it forward a little more.
I measured it.On my remington from the front of the front sight..to the front of the rear sight is 16".That is on a 26" barrel.I mount it the same on my 870 super express mag.That distance was 18 1/2"(1/2" more assumably because it has a 3 1/2" chamber..).I like how small the front bead looks on my longer barreled gun.It makes pin point aiming so much easier.Hope I helped.