It's annoying, isn't it? Some patterns do tell you the actual measurements, so you can figure out how much ease to take out, but I really wish they'd just make them the size they say on the envelope so you don't have to do complicated adjustments as you go.
Is the waistband in two pieces with a seam in the middle? I'd be inclined to shorten it at the centre rather than the ends if the ends are already finished.