My experience has been that most of the time a double rifle will shoot a bullet that weighs approximately 75% of the weight of its regulated bullet weight to the same poi over the same charge of powder.
It's worked for me in the following calibers:
470
500/450
450/400 3"
450/400 3 1/4
For example, in my 500/450 Holland. I shoot a 350 gr Hornady RN to the same poi as the Woodleigh 480gr soft over the same charge of Rel 15.
My 450/400 Jeffrey shoots the Hornady .411 300gr bullet to the same poi as the 400 gr Woodleigh soft over the same charge of IMR 4831.
When I got my Paradox and found the load that regulated well I stuck with it. I don't find the Paradox difficult to shoot, even from the bench since the slug doesn't engage the rifling until the last inch or so of the barrel.
Unless of course, I misunderstand your question and you are asking about a target shotshell load.
