Would really like to give Gustin's button-downs/workshirts a shot, but struggling with the fit. I think others in the thread have also mentioned the need for an "athletic" cut, so I just wanted to be one more voice for that.
Looks like currently my best bet would be to order a standard cut medium (I have a ~42"-43" chest), but I have a 31" waist, so I end up swimming in a 41.5" waist cut. I've had to overpay for custom shirts to date, but would love an alternative that had more taper in the cut.
I think you will find the classic medium to be too small on a 42-43" chest. I also have a 42-43" chest, as measured by professional tailors for my suits, so its an accurate number. And I tried a Gustin classic medium and it was too tight around the chest. Which isn' surprising based on the fit guide. You need 2-3" of extra shirt fabric around the chest, added to your chest measurement. The Gustin classic medium is specified at 43.5" chest so a 42.5" chest will be a very tight fit. My test classic medium almost fit but there was some unnatural tension at the upper button holes causing it to lay wrong, and that was prior to washing. It was likely to shrink a bit over time even with air drying, so it would get worse not better.
I returned the classic medium for a classic large, This shirt fits my chest well, consistent with the fit guide 45.5" vs my 42.5" chest. Some of the other dimensions are a bit too big on the large but that is less bad than having the chest be too small which simply doesn't work.