What do you want the pattern to look like?
Are you a “count the 10 ring” guy or a hunting pattern guy?

I don’t have an 870 20, but several guys I shoot and hunt with do. Some are on here and maybe they will chime in. The best long range patterns I’ve seen are from a .555 IC and Hal’s MOA load.

A lot of folks like the 1 5/8 oz loads and Carlsons 575 or 585 chokes for a more open and even pattern.