As I'm doing new Travmap rolling (don't know when, it seems I rerolled my old Travmap on my barb which I don't recall happening), so I've decided to estimate chance for a minions to spawn in certain region based on measuring whole area where minions can spawn from wall to wall.
I have sloppy estimated chance to spawn "ideal" map layout for Travincal, aka when all minions spawn in this region alone:
If you get ALL minions in this region, you got really great map. Minions have 3 spawn locations, the same as number of bosses since their minions spawn with them at same place. All 3 spawn locations must fall withing this quadrant.
Bad news is, in best case this kind of map spawns once in ~400 rolls. Since it is possible to roll 4 maps per minute, it seems at average it would take ~1 hour 40 minutes to roll such map, but ONLY if you can roll maps once in 15 seconds (means immediate s/e upon teleporting, NO killing whatsoever). If you can roll 3 maps per minute, estimated time rises to ~2 hours 10 minutes. You get the idea.
This is actually really strict estimate and I can't confirm is it like that in practice. I have an idea how to better estimate that by observing how often will I see minions spawn in designated area which could lead to better estimated time. Honestly, 1 hour 40 minutes seems a bit too long from my experience. Also measuring area based on screenshot is really sloppy work. Also I don't know do minions preferably spawn in some areas more than in others, I just assumed they spawn randomly across Travincal area (not whole, it is pretty clear where boundaries are with some map rolling).
EDIT
Did 50 map rolls, counted 19 bosspacks spawn in that area during that time. It is too early to say and more tests need to be done, but by this numbers, chance to roll map with all minions in designated area is ~1/313, or if you roll 4 maps per minute, that will take 78 minutes to roll at average. But, I'll do more tests to see more precisely. In this 50 rolls I had 2 very edgy cases I dismissed as not being in the area. Every minor change in chance to spawn there has dramatic change in overall probability to get ideal map. For example, if I counted 17 minions instead of 19 in 50 rolls, that would put estimated chance to roll ideal map at ~1/460. It is clear plenty of tests should be done to have any good estimate, this is too unreliable so far.