Sorry to say Twitchy, there just isn't any hard equations or hardened rules on plain bearings. It's, whats the application, whats the ambiant temp, what is the lowest speed/highest, what is the shaft loading, what type of material will be transported, what is the horsepower requirements, what temperture will the machine rise to, what type of oil delivery system would you have, what viscosity oil will you be using and why, what did you have for supper last night and what time will you go to bed. I think you got the drift. But start with a least 0.002" (TIR) and work up. You need an oil wedge to support the shaft, so, to tight will cause rub and too large will cause oil/shaft whirl. Depends, depends, depends...
Cheer up, it could be worse, it could be me trying to figure this out.