One easy way is to use a geared bike and find a combination thats either near the new gearing you've chosen or you like to use and base your gearing choice of that.

Sheldon brown has a good gear inches calculator so you can see how it would match up:
http://www.sheldonbrown.com/gears/

I use a 48x18 in SF and its pretty good all round. Good speed on the flats, but with enough left to get up most (but not all) hills without a huge struggle. Occassionally I still need to push up the odd block, like the Fillmore St hill. Haven't yet had a problem spinning out.