I finished making a second Negroni muslin a while back and I almost can't recollect all the details. Let's see. I traced a medium but added length to the body and arms. I traced the largest sized cuffs and tapered the sleeves out to match. The biggest change I made from the original pattern was to change the collar from a camp style collar to a traditional collar and add plackets, which I did using Peter's instructions.
The overall fit is much better in the shoulder and the neck fits perfectly. The shirt still creases near the armhole but it looks better than the large muslin in the front and back. I had to use a 3/8" seam allowance to sew the shirt front to the back and at the armhole, since it was a little too snug.
The fabric was labelled as a chambray but it really is an oxford.
Things to remember for the next time: the shoulder, collar, plackets, cuffs need no change. I need to shorten the collar stand I drafted by about 5/8". I need to sew and press the cuffs more carefully, so that the inside fabric doesn't show! I would add even more length in the body, maybe shorten the sleeves a little. Most importantly, I need to trace a larger armhole, with more room in the shirt body.
It isn't perfect yet but it's getting closer with each try!