It has not been quite as long since I made a shirt for me as it has been since I made a skirt like I posted yesterday, but it has been a while since I made a shirt for me from McCall's 5675 (1977)
...now...I am currently nowhere near a size 12, but I realized that, since one of the features of this pattern is the gathered neckline, it would probably work out perfectly fine if I just...added two inches to the centers front and back, increasing things 4" total. I did that on the fly, by using a see-through ruler to move the pattern center an inch from the fold on both sides. (Also pictured: why get proper pattern weights when you have so many extra pairs of scissors anyway?)
And it worked!