I've never mounted an Ofmega crank on my bikes. I have not seen any charts showing recommended lengths for spindles either (IIRC, even Sheldon Brown does not provide specs and numbers for Ofmega BBs). I suspect that they might be similar to what is used on other Italian cranksets. Something like 113mm to 116mm for 68mm BB shell widths. Maybe someone else with more Ofmega experience can chime in to confirm.

Chombi