I finally had a chance to look at this. Danny, did you try to cut the joint with the 11/16" spacing with a 1/4" depth (what Incra suggests)? I would think you'd end up with kindling, too. Here's the long-winded explaination:
It's been a while since I've looked at dovetails. A while back, I had checked them against the "Master Reference Guide", and I knew pyRouterJig matched at least a few of the 7 deg. and 14 deg. cases. I own those bits, but not the 9 deg., which I admit I never had checked.
So I tried all 10 of the equally-spaced dovetails in the Guide. pyRouterJig matches all of them exactly, *except* Danny's case: 9 deg., 3/8" width, 1/4" depth case (the "bad case"). Of the 10 cases, it's interesting what is unique about the "bad case". The exact dovetail spacing is given by
exact_spacing = 2 width - 2 depth tan(angle)
I get 0.671" for the bad case. Because the Incra can only position in 1/32" increments, this number must be rounded to the nearest 1/32". I get 21/32", while the Guide somehow gets 11/16". Note that
0.671 - 21/32 = 0.015" (pyRouterJig)
0.671 - 11/16 = -0.017" (Guide)
That is, roughly 1/64" in either direction. Note that in either case, this difference is a lot larger than any of the other 9 cases, which typically need to adjust the spacing less than 0.004". I considered that the Guide "rounded up", so maybe it does that for all cases. But no, this case and the 7.5 deg. case are the only two that the Guide rounds up.
I'm not arguing that pyRouterJig is correct. It shouldn't even let you specify this depth for this bit, given how the Incra works. As Danny found out, you get kindling. But I would be surprised if the Guide's 11/16" spacing is any better; if anything, I'd bet it's even worse. But maybe I'm missing something. Maybe my formula is wrong above, and I'm just getting lucky for the other 9 cases. If so, please enlighten me.
For through dovetails, I need to figure out how to restrict the depths so that the error in shifting to the closest 1/32" is small enough. Danny, I like your idea for half-blinds. I'll get that in at some point, too.