Sewing, like knitting, is sometimes a real adventure in fitting. Even if your sewing is impeccable, if the garment doesn't fit you, it doesn't look good. A simple thing like fabric stretch and hand can really compound the issue. I'm sure we have all gone through this scenario--like the pants in this fabric, but in this other one, they bag to my knees.
Christine Johnson, a knitwear pattern designer, has actually produced a handy little page to help with this problem, based on ease and stretch of fabric. After thinking these things through, you pick your pattern size (or, adjust up and down if you are working with your own tnt wonders) based on the final fabric measurements to give you the ease, and fit, you desire. Check it out:
Perfect sizing worksheet