Bench rest the gun and shoot at a target @ 13 yards. Don't worry about your comb for now. Sight down the rib using your desired sight picture, which to me should be a perfect figure 8. If you want the gun to shoot "a little high" get it to shoot 2" high at 13 yards. If it hits too low, raise the back of the rib and lower the front. After you get your point of impact to where you want it, adjust your comb so your cheek is comfortably on the stock....how you want it, giving you the sight picture you had when bench firing your gun. That's how I'd do it.
hairy gave a good method for most guns, but not the K-80. The rear of the rib does not move, in fact the front of the rib does not move. When the adjustment at the front of the rib moves the rib closer to the barrel, the barrel actually bends up. Bending the barrel up does not change the sight picture.
Pat is correct. On the K80 the unsingle has an adjustable POI via the rib/barrel adjustment, the under barrel has a hanger to adjust POI (you must physically change the hanger to change POI) and the Over barrel of the O/U has no adjustment other than the comb, if your gun has an adjustable comb.
Here's what I do. Assuming you have an adjustable comb, I adjust the comb so that I have the sight picture I want using the unsingle. I adjust the barrel adjustment so I am happy with how it breaks targets. 60/40, 70/30 or 100% high, whatever you like. Keep adjusting the barrel adjustment until you're happy. Then I go to the patterning board and pattern the unsingle until I am confident that I know what it is shooting.
Then I put on the O/U barrel and pattern the over barrel. I now adjust the comb up or down until I am satisfied I am shooting the same POI I had with the unsingle. Once you have the unsingle and Over barrel shooting the same POI do not change the comb any more. This also assumes you want the both the Over barrel and unsingle to have the same POI. Most people do.
Pattern the under barrel and adjust the POI of the under barrel by selecting the correct hanger if the one you currently have has a POI that is too high or too low.
If no comb adjustment was made when patterning the Over barrel, then the unsingle should still be shooting where you had it. The Over barrel and unsingle will have the same POI. You are finished.
However if you changed the comb height to adjust the POI of the Over barrel, you'll want to reinstall the unsingle and adjust the barrel adjustment, to either raise or lower the POI, so you have the same POI you had before you changed the comb to adjust the POI of the Over barrel. At this point the unsingle and the Over barrel will have the same POI, the comb will be in the same position regardless of the barrel you are shooting and your sight picture will be the same on both barrels.
Yes you can do that, but most of us shoot the unsingle considerably more than the O/U, so I take this approach to make sure I'm real happy with where the unsingle shoots. Then I adjust the Overbarrel to the same POI and readjust the POI of the unsingle if I moved the comb.