I have owned 3 combination guns in the past. Two regulated fine, one was impossible...

I'm curious as to a few questions on how to regulate a combo. I do not own one now but want another. I have hunting seasons that overlap and have a great need for a combination gun though I have no idea where to start looking for one!

Anyway, here's what I did {maybe it is not the right way to aproach it?}:

I fired one round each of bird shot, buckshot and slug at a target 25 meters away. Fortunately, all three had essentially the same POI in relation to POA. Reason I shot the smoothbore first was that all birdshot patterned same POI and I had no way to adjust loads to move POI.

I then set out to zero the rifle barrel at 100 meters, hoping the relationship between the shotgun and rifle barrels allowed for close proximity of the two at 25 meters. Elevation was not a problem, but in the case of one gun, windage was, and it meant that in order to get the rifle barrel anywhere near the center of the shotgun pattern, the rear sight was hanging off the side of the gun.

So...

What method is best used to zero combo?

What principles of handloading are there {if any} to adjust a rifle barrel to move windage in a combination gun?