the main reason i suggest 310-320, is that everyone knows that you go down a size running the same fuel, but if you are buying local pump, it changes every 2000 ft. and makes you go up 2 sizes or more. so basically, if you ride sea level, but make a trip to the pass @ about 1750 and up, you will be fine by dropping a size. when you use up your fuel and need to buy more, the local station will have fuel that is formulated to atomize differently for higher elevations, which, in short, calls for 2 sizes larger mains than the sea level fuel. this is one reason why gas pumps have a disclaimer "not for use in aircraft engines"
not sure if i explained that right, but it comes out to about to +1 per 2000ft or so, on pump