Long time lurker first time poster. This forum has been very helpful to me in the past but right now I could use some direct advice. I've used google and the forum's search function to find MOB trap/farmer/grinder tips but most of what I find is "Build this..." or "I improved ____'s design." and very little raw data. The basics of a MOB drowning trap (dig ditch, flood ditch, ????, profit) are incredibly simple but it's the actual MOB spawning information that I seek.
1) What is the smallest area necessary for a dark room to spawn MOBs?
2) How close do you need to be for MOBs to spawn in darkness? (I read 50 blocks somewhere but I am unsure.)
3) What block should I use to encourage the MOBs to spawn? If I use soil will the non-hostile MOBs spawn as well?
4) Will MOBs spawn in shallow water or do they only spawn on dry blocks?
I am attempting to turn a multi-layered strip mine into a drowning trap (complete with loot drop zone). The mine shaft is a 5x5x25 hole that continues down past 4 layers and stops at the 5th layer which is larger and will be the base (safe room) of the trap. Each of the 4 middle layers is 3 blocks high and 5 blocks deep all the way around the mineshaft. I understand that these layers are a bit small for spawning rooms but I thought the fact that there will be multiple rooms will help (yes? no?).
I don't know if this image will help but here is what the mineshaft looks like from below:
I've built a MOB drowning trap before but that was a copy from a Youtube video I watched. This will be the first "custom" trap that I've made and I don't want to block off and flood these rooms only to find out that I've failed (no MOBs spawn, loot doesn't flow down, spiders get clogged, etc). Thanks for any help you can provide (even if you just link to other threads on the subject that I might have missed).
TL;DR What is needed for basic dark room to spawn MOBs? How to make them fall into drowning trap?