The final test I preform for my double rifles is to get two animal targets, here in the USA they are usually deer, and set them up at the distances I intend to be able to shoot and hit something with the particular double/load in question. Then I start out kneeling and shoot the right bbl on the right target and the left bbl on the left tgt. I usually shoot 6 rounds. This lets me know how the rifle is going to perform in the field. I also test my folding leaves at 100 yards and consult ballistic trajectory charts to know if they are close.
My 450 No2 is regulated for 75grains of cordite/480gr bullet and shoots best with

81gr. of IMR 3031 350gr Hornady RN
81gr. of IMR 3031 [80gr with solids] 480 and 500gr bullets.
100gr of IMR 4831 with soft and solids.
88gr of RL-15 with 480 and 500gr soft and solids.
The 480's are Woodleighs, the 500gr are Hornady's and Swift A-Frame's. The only bullet my double does not seem to like is the New Hornady Encapuslated solid, but I have not experimented with it much, it might just need more or less powder.
RL-15 loads seem to have the lowest pressure.
The 350gr. Hornadys also shoot good with the two other powders, but I use 3031 for them because I have a lot on hand. All the above loads "regulate" well and hit with the sights at 100 yards [on paper] and at 200 yards on my "200 yard rock". My double seems to slightly favor 500grain bullets over 480gr. So far the 500gr Swift shots the best. Next time I buy Woodleighs I am going to try the 500grain soft and solids.
I always use filler. It does not seem to make any difference if I use polyester pillow stuffing or the two different types of foam I have. I prefer the foam as it is easier to load and does not make a mess on the ground.