Does your 870 have a feature to adjust the butt to make the gun fit you? If so, you may be further ahead to make that adjustment. Then practice to perfect your mount. That may be all you need to do.
If you intend to shoot slugs thru' it, I'd add a rear sight with a large enough aperature to allow for low light conditions, then adjust the butt after shooting for POI (point of impact). The greater distance between the front bead and the sight will give you a degree of greater accuracy over a front and mid rib bead.
Shotgunning is all about gun fit. A comb too high will give too much space between the two beads; too low will give too little space. If you can't "figure eight" because of poor stock fit, then you will have to remember the sight picture needed to compensate. But..........
You are using the shotgun as a rifle where you can mount the gun deliberately so I feel the rear sight is best for your application unless you will be using it for upland birds.
If the cost of a sight is out of the question:
Pattern for POI, pattern again on another day....pattern again. Remember to do this from the position/distance you will most likely shoot. Then adjust the butt so the gun shoots exactly where you want it with a flat rib and sighting down the middle of the rib.
Now, you can add a bead to the "back" of the receiver if you want, note the sight picture and align the beads the same way each time. The back of the receiver eliminates the worry of drilling thru' the bb'l.
Remember the axiom: 1/16" of butt correction will equal 1" of POI change at 16 yards. Naturally 1/8" of correction will = 2" at 16 yards or 4" at 32 yards. Take the butt "up" to raise POI and "down" to lower.
Cast (right/left), if needed will require you to use a rear sight since you will no longer be looking down the middle of the bb'l.-Jerald