I've started experimenting with an SMP mob spawner/killer system and the results are a little bit strange. About my system: 2 floors of 17x17x2 and 17x17x3 blocks, completely dark, with some water tunnels to get mobs to the killing floor. The exit of the system is farther than 24 blocks but nearer than 144 blocks. The tunnels were mistakingly built with the width of 1, so if a spider spawns inside the system, it cannot go anywhere.
So this basically works, but the spawning of mobs is in no way systematic! I am almost always watching them with the F3 button and can see that sometimes a lot of them spawn, and that especially happens when I move around a lot, running, doing my own things and so on.
But if I stand on the same place, for like an hour or something, it is like they spawn in the tower for first 5 minutes, and I get the loot at the exit, but for the remaining 55 minutes, nothing happens!
So what other considerations should I have besides being farther than 24 but nearer than 144 blocks from a spawning floor? Does it matter if I move, where I look, what I do? Does it matter if the game window is minimized or not?