I used CSL's method of using a small round straight pin and used grease to make it stick on the rib. It looks like 14" from the muzzle is best for a figure 8 sight picture on a 28" barrel. Distances can vary due to comb height and facial dimensions.

I have one such Parker and the distance is 18 1/16" center of bead to center of bead. The research letter indicates special instructions for "...2 Lyman sights..."

Ted, what's your barrel length?

This gun has 28" barrels.