Another good link! Thanks.
Couldn't find anything in stock in town for the rear of the WRX today. By that chart I think I'll need at least a 5.5/6 tube for my new 150/60 tires. Unfortunately due to getting a flat today and then having to ride on it a bit, I definitely need a new tube. I have a 5.0/5.3 on the shelf. I'll have to try it and see.
If I ever manage to get the tubes I'll also post a review of the Conti SM tires.