The well-fitting sweater I originally measured was an XL. I'm looking for proportions for the armhole and shoulder shaping. It's meant to be informal businesswear, so that it will go with chinos and a collared shirt, and I think unshaped shoulders are a bit rough for that look. (I've looked at Knitty's Petrol, and googled around for men's sleeveless sweaters, but they either don't have shoulder shaping or they don't have schematics.)
The sweater is designed to be 23" across at the underarms and should be about 26" long from the bottom hem to the top of the shoulder (or maybe to the top of the middle shoulder.) I have at this point knitted well into the armhole shaping, so it's 16" from bottom to the armhole shaping. How deep should the armholes be? and how deep should the shoulder shaping be? I realize that measuring either my Dad or his sweater would be the best step, but the truth is I'm secretly hoping to surprise him with the finished sweater the next time I see him.