Talk to Vicki Keen of Keensights (http://www.keensights.com). She has done several ribs for me, all perfect, no subsequent problems, and the price is very good.
She does it by manufacturing an aluminum rib specifically to fit your gun and then attaches it to your gun by a combination of screws (using the bead screw holes already on your gun) and something like RTV. (You can't see the RTV when she is done.)
I recently compared one of her ribs (on my gun) to a Moneymaker rib (on a friend's gun) and found no reason to prefer the Moneymaker rib. While opinions might differ, I though her rib looked a little nicer than the MM. It is also easily removable, if you want to sell the gun as pristine.
Quite a few people posting on TS.com have spoken favorably of Vicki's product, and I don't recall ever seeing any indication of dissatisfaction.
Dave Berlet in New Knoxville, Ohio makes add-on-ribs that are the best I have seen. Very high quality and shooter friendly prices. YOu can call him at 419-753-2557, Tell him Big Papa told you about his work. I does not cost to call and get some good information.
The POI of a gun is set by the height of the comb. The height of the comb is affected by the anchor point you use. I have very high cheek bones. Therefore, a gun that shoots 60/40 for you might shoot 40/60 for me. If you were to shoot my Trap gun, it would probably shoot 120% high for you. I have it set for about 90% high for me.
Talking about POI, it just dawned on my that we all talk about percentages. But shouldn't we be talking in inches? Like how high a gun shoots based off of 13 yards? Something like a gun shoots 3" high @ 13 yards.
Whether you talk about POI in inches or percentage depends on your point of view.
Percentage looks at the whole pattern, and inches looks at the center of the pattern. It's really the same thing. Either method is difficult to measure. First you have to measure the pattern, then determine where it is in reference to the Point of Aim. Since no two patterns are alike, this measurement is not easy. Even at 13 yards, it is guesswork to determine the center of the pattern or the edges by just looking at it.
Now, if you use some software like Shotgun Insight, you can be more precise. It takes every pellet strike into account and mathematically finds the center of the pattern. Most people won't take the time to use the software.
So, X" high at 13 yards or XX/XX pattern are both OK ways to describe the POI as far as I'm concerned. They both indicate where your pattern is in relation to POA.