Yeah, I'd go ride around in the grass someplace and see how comfortable you feel. I think the most important thing, gearing wise, is that you're comfortable riding around on a flat area for an extended period. Any terrain that's too steep to climb will probably have many other folks off the bike and running as well, and there doesn't tend to be any really extended climbs in cx racing to my knowledge. I'd try out the 44x18 and see how that feels, but you might wind up with something slightly easier.