NitroX
I tried to e-mail you some additional info, but maybe it did not make it.
Maybe this will help. ADI AR2209 is slightly slower burning than RL-15. I would start with the recommended load for RL-15, and substitute ADI AR2209 and just work up until the load regulates properly in your rifle. Watching for pressure signs of course. Do not forget with the big case of the 450 No2 you must use a filler.
You could also use ADI AR2213 as it burns just a little faster than IMR4831, but all the experts seem to favor RL-15, so I would start with AR2209.