First let me say that I'm not extremely knowledgable on these things, but I seem to remember my brother (a licensed journeyman electrician) telling me that the "rule of thumb is 1.5x the rated current for startup.
This much I do know: if there are 2 amp ratings, there will be 2 volts ratings as well.
Look closely at the data plate, it may say something like this:
VOLTS 240/120
AMPS 6.2/12.4
The 6.2 amp rating is for 240v, the 12.4 amp rating is for 120v.
Personally, I would figure 1.5 x 6.2a = 9.3 amps starting current for 240v OR
1.5 x 12.4a = 18.6 amps for 120v, and size my conductors according to the startup amps.
These numbers are just an example, and may or may not correspond to your particular motor. The only way to know for sure how much your motor draws is to slap an amp-clamp on it while it's running. Starting current may indeed be 2x running current, but since it usually takes less than half a second to get an electric motor up to speed, 1.5x the nominal current is usually sufficient for sizing your components, i.e. wire, ckt brkrs, etc. Hope this helps!
BrianH