For the last over 20 years I used 150g Partition over 48th of H4350 using Remington brass, in my FN commercial 98. It shoots these around 1.25" . I've added a barrel band sling swivel canvass sling and express sight. Now I have a 'retro Rigby.' I ordered some Hornady Rigby brass and 154 spire points. For what I do now, deer and pigs under 200lbs and 200yds they will work as well as the partitions.
My first question is: how much,if any should I back off to begin with? FWIW, its very long throated.
Any other opinions.?
captdavid

Aren't these the exact same case with a only a different name?

He's kinda, but two different manufacturers, Remington vs Hornady. captdavid

Remington cases are very often a bit lower capacity than most other brass. That said, it is never a bad idea to back off a couple of grains and work back up. There is no downside to this practice, and every once in awhile it avoids a problem.
Never know where you will find the new sweet spot.

In a situation like this I'd back off two grains (46gn H4350) with the change in cases and bullet and load three each of 46gn, 47gn, 48gn and 49gn. I wouldn't be surprised if you may reach 49gn H4350 with the Hornady bullet to get the same velocity as you got with the 150gn Partition with 48gn. If you get to 48gn and are happy just pull the three 49gn loads down.