For those not clicking the link, we're talking about monsters listed in Monsters & Treasure as appearing in numbers like 10-100, 20-200, 30-300, and so on, the immediate example being bandits. Delta's asking about what dice most GMs use (he's been asking a lot of survey questions lately, to get a better idea of how many techniques are out there in the wild.)
But I'm thinking more about what that number is for. As many people will tell you, the numbers listed on the Monsters & Treasure table are meant more for wilderness hex stocking than for dungeon stocking or random encounters. I'm pretty sure neither Gary Gygax nor Dave Arneson ever did something like:
Gary/Dave: "As you enter the forest, ahead you see..."[Stops to roll dice]Gary/Dave: "300 bandits."
I mean, first of all, why would they wait until the last minute to work out an encounter of that size? Why would they pit that many opponents against 1 to 6 PCs, an equal number of retainers, and a handful of miscellaneous hirelings? Why wouldn't the PCs detect what is basically a military camp until they were almost in the camp itself?
Aside from obvious references in the rules themselves that contradict the above hypothetical example, and people who actually played with Gary or Dave reporting that they didn't roll for monster numbers at the table, but picked a reasonable number from the listed range, we have to take into account what you would even need this number for. Wandering monsters are typically in multiples of 1d6, modified for party size and monster level, so we don't need it for that. What we need it for is:
- Approximate encampment or lair size (30 bandits have a couple tents, 300 bandits probably have a small fort and a couple buildings.)
- Guide to encounter size (patrols follow wandering monster rules, deliberate response to threats will be up to a third of the full force, major expeditions will leave at least a fifth of the force behind to guard home base. Also useful for mass combat encounter designs.)
- Total pool of available monsters (PCs should stop encountered bandits when the bandits have been wiped out.)
- (For bandits, goblins, orcs, and other structured groups:) Calculating leader types present.
So really, it doesn't matter how you get to that total number of monsters in the group, or if every possible number within that range can be rolled, or has a probability that fits into a linear or bell curve distribution. You just need a number beforehand so that you can make broad decisions like "What's the camp/lair look like?", "How far away can it be detected?", or "Have they run out of cannon fodder yet?"
What that means to me is that we don't need to figure out what die roll best fits the listed range. Instead, look at the maximum. Is it one significant digit? Rounded to the nearest ten or hundred? Roll one die to determine that first digit. For a range of 20 to 80, roll a d8, treat a 1 as a 2, and multiply by 10. For 30 to 300, roll d6/2 and round down, multiply by 100, and treat a 0 as the minimum. If you really don't want a round number, roll d10 or d100 for the last one or two digits, if the number isn't already at the maximum.