My Purdey 20 bore rifle runs 2-1/2 drams, my lighter 16 bore rifles run 3 to 3-1/2 drams, the WR 16 bore rifle seems to like 5 drams.

Two and a half for a ten bore rifle is a waste of time and powder.

Also, don't bother with anything but known-good black powder lube in your black powder bore rifle loads. You'll do well with lots of it.
You want a soft and pliable -even moist- fouling for consistent shot-to-shot performance.
Leave it dry and you'll frag those wad cups, killing any chance of accurate groups.