Eric,
You won't be able to come up with some kind of table to tell you what the bullet weight changes do to the regulation of your rifle, there are just too many variables. There is a bunch of conventional knowledge around this wedsite, some of which is that you can't shoot more than one bullet weight in a double rifle and expect to have any kind of accuracy. It has been my experience that you can, and I do shoot more than one bullet/type in some of my guns. The more intense your caliber, the more the POI/regulation is effected by changes in bullet weights and speed changes.
I have found that I can vary the weight and or type of the bullet used as long as I can get it up to around the same speed as the original bullet. Recoil impulses may also be different for different powders or amount of the same powder, so some "tweeking" of the load may be needed after you acheive your target speed for any certain load.
The bigger the bore, the easier I have found them to be to regulate and change bullets on without radically effecting the outcome. Also, if going to a heavier bullet than normal for your gun, you may not be able to get the speed up to the level you need without going over the limit of safety built into the rifle. Acceptable accuracy is relative also to the game you are hunting. What would be good accuracy for elephants might be totally inadequate for deer. You need to decide what you are willing to work with.
In my opinion, any rifle that will put two rights and two lefts into a fist size group, at the distance I am going to hunt at, is an acceptable gun to hunt with at that distance, but might not be useable at a longer distances. Double rifles are really fairly short range hunting weapons in any event, say 50 to 125 yards. They can be extended out further with the addition of a scope, but most "purists" don't like to do that to a nice double. I know that I am going to get alot of flak over this and we will hear all about the 300 yard shot some guy made on an antilope or some such, but in reality most people have no business shooting offhand at over 100 yards on live North American game. Bob