Swampthing: I usually weigh my shafts before and after tapering as well. I usually get 15-20 grain reduction in 11/32 shafts with a 10-inch taper to 5/16. We're in the same ballpark. I maybe take mine down a fraction more.
3Feathers: as others have pointed out, you specify the physical weight you want when you order/buy the shafts. My numbers generally concur with Bjorn's. Full length 11/32 cedar shafts can run from about 350 grains to about 440 grains. I've had a few in the 460s and even one or two that were 500 grains. You can order from the low, mid or high end of that spectrum depending on the weight of the finished arrow you want. If you order them already tapered, you'll pay a little more but get the exact weight you want. If you plan to taper them yourself, plan to lose about 15-20 grains per shaft. Not enough to worry about, IMO.