Like all patterns, I measure the pattern before making a muslin. Usually I go down, but not always. I take into account the fabric too. In this case a very stretchy (but clingy) rayon knit was used and I went by the back measurement (across the back at mid-armhole) where I wanted to be 'fit'. I didn't want dropped shoulders.
Love, love, love the way the cowl facing gets folded into the back self-facing. Very slick. Fast, fun pattern to make.
Un-tucked with a nice deep hem--which you need in such a light-weight fabric to allow the top to drape properly and not curl up.
I mostly wear it tucked in, or with a belt over-top.
Now please keep in mind, I've dropped over 10 lbs since I've made this (happy dance here) so it may look a bit big. The only thing I don't like (and that's just part of the style, not the pattern drafting) is when you bend over...hello girls to the world!
I made the "blonde's white top" with the shorter sleeves---which are a bit longer than the drawing, but I like them.