You can build a 16x16 grid of your nearby area and wherever there is the most spawning poof...there you go. Or...what I did...

Build your huge massive mob tower, let night fall and hope they spawn on your platform...go and watch if they spawn, turn to your right (about 20 blocks away) and see 8 skeletons, 4 zombies, 2 creepers and a few spiders and go...FUCK...wrong area...


I let my SP game run while I was AFK (about 5 hours and I came back to 7 feathers, 4 gunpowder, and 3 arrows... :wtc: