Hey pals, here's my first "guide", more of a little tidbit of advice ...
I am gathering spawndata for certain spots, farm them until i have statistically accurate numbers and then post them here to help you pick the best place to hunt, not be frustrated as fast or hunt at the best times. Spawnrates are not only influenced by the overall rarity, but also by competition, many abundants make the rares even rarer ... or do they :D? That's what you can read here ^^
First i started with PKMN Tower Underground, since i like Litwicks and wanted some:
Time : Day
1000 Encounter
statistical accuracy = 1%
Rattata = 618 = 61,8%
Zubat = 176 = 17,6%
Gastly = 99 = 9,9%
Misdreavus = 63 = 6,3%
Shuppet = 35 = 3,5%
Spiritomb = 4 = 0,4%
Litwick =5 = 0,5%
Time : Night
1000 Encounter
statistical accuracy = 1%
rattata = 629 = 62,9%
Zubat = 134 = 13,4%
Gastly = 87 = 8,7%
Misdreavus = 73 = 7,3%
Shuppet= 41 = 4,1%
Duskull = 13 = 1,3%
Litwick = 11 = 1,1%
Spiritomb =11 = 1,1%