Thanks Henry, TTailor and tombennett for sharing your thoughts on patterns. This is actually the third pattern I've considered, though it's the first to make it to the sample stage.
The first pattern came from a supplier that caters to Amish communities (similar but not the same as the potential customers). Intended for home sewing, it was extremely squarish, with even wider shoulders and scyes about two inches too deep. What they are wearing today amounts to a classic modern style RTW jacket with a Mandarin collar. I thought I could do better and this would probably be too "Grandma Moses" for them. Availability of these patterns is also sporadic, so I preferred to provide my own.
Having some experience with F. T. Croonborg's 1907 "Blue Book of Men's Tailoring" I next drafted up his "Policeman's Blouse", which is a sack coat body with a standing collar. The resulting mockup fit me fairly well, but has the opposite issues: very close fitting and with a high and tight armscye. Based on what they are now wearing, I think they want something easier fitting, and I am hoping to be able to work with standard sizes.
The 1970 drafts were available on the old Forum before it went down and seemed to be well thought of by some of the regulars. I selected the sports jacket as the less shaped of the two. I did the pattern draft, made a mockup, and thought I had found a happy medium; somewhat shaped but not tight, with an "easy" scye but not too deep. I didn't realize there was an issue with the back until after making the sample and taking photos.
The size chart (supposedly also from the 1970 T&C and taken from the old Forum) shows a half back measure of 8-3/4 inches for a 42 regular. I applied this on top of the 1/2" for "drape" allowed in the pattern, plus two seam allowances of course. Is this in the ballpark or much wider than I should have done?