I do, but I just approximate it depending on how difficult I found the encounter. What do you think would make a good rule, 3-4 'sub-minions' per minion?
I have so far only rolled out these sub-minions four times and I don't have a great handle on judging the difficulty yet. The second time I threw them at the party (a horde of bow wielding goblins) the party was quickly overwhelmed. The ranged attacks hit them hard. I think that next time I want to simulate a hail of arrows like that, I'll do it as ongoing damage (maybe with a save for half damage).
Yeah, I also do this: I use 'mooks' when I want a quick skirmish rather than a horde, using a simplified version of the hit box rules. Go down in 1-2 hits, but do standard damage.
And while I really liked minions as a DM at first, I quickly found that my players hated them--they feel cheated of their damage rolls. But they don't seem to mind the sub-minions and mooks.