Ales: 62°F 75°F (17°C 24°C)
Lagers: 46°F 58°F (8°C 14°C) *Note: Lager fermentations can be started warmer (~60°F, 15.5°C) until signs of fermentation (gravity drop, CO2 production, head formation) are evident. Cool to desired fermentation temperature once signs of fermentation are observed.
Wheat and Belgian styles: 62°F 85°F (17°C 29°C)
Got this from wyeast, Seems like a very broad spectrum. Pick a middle point, calculate a high and low standard deviation and bobs your uncle you got a perfect fermentation!
With a temp swing like this i can see a +/- 3 degF is a nice and tight control.