Jos - you could take the gun to a smith and ask them. I did that with my great grand daddies sxs that bored at a 16 ga.

He suggested a load and told me not to exceed 110 grains.

I don't trim my shot cups. but I look for shot cups that I can fill to the top with my chosen load.
I am like you at 80-90 grains as I found this patterns best. But I need time at the patterning board and will report back.

I have #4 copper plated lead to pattern as well as this new ITX shot (supposed to be like Heavy shot)

I got some new shot cups in too from BPI as I am trying to improve my pattern.

JW