The old standard is equal measure of shot and powder. So, if you are using say 90 grains of FFg, use that same volumetric measure for your shot.
Then try it out on a pattern board and see how it patterns. If there are holes, try adjusting something, but only one thing at a time to produce a more dense pattern.
I am sure there are folks more knowledgeable about the intricacies of m/l shotguns, but that is a place to start.
Nothing about this sport is rocket science, mostly just a few basics then add on from there with your experience.
Have fun,
John