Above FL180, there is spacing of 1000' between each level. Westbound gets even, eastbound odd. This lasts until FL290. At that point, there is 2000' spacing with the even that would have been at FL300 getting the first 2000' jump (over FL290) making it FL310. The next westbound, therefore, would be FL330, even at 350, etc.
Occassionaly, for weather or traffic, you will can be assigned a flight level that is normally for the opposite direction - and usually only temporarily. You will hear the ATC assign "flight level 350, wrong way" just as a comfirmation that they know they are putting you on the opposite direction flight level.
Dave Mark - Intrinsic Algorithm - Reducing the world to mathematical equations!