I would prefer the rear axle to be further forward in the dropouts--I guess that is a compromise in chain length to accomodate the two cogs. I run a 46/18 drivetrain on my Wabi Special, and have 700 x 32 tires. The rear axle is closer to the middle of the dropout slot, and I use a chain with a half link so that the axle position and tire/seat tube clearance are both good.