Whenever I have a shirt with a non standard collar opening, I place it over a standard shirt , flat, on my folding table.
I then easily feel the standard collar below, and measure, either with my fingers or a ruler.
Had to do the same thing the other direction with mock turtlenecks.
When there is doubt, I have also been known to cut out a paper print, have my wife try on a ladies shirt in front of the miror, and tape the paper where it looks good.