OK maybe I misread that. I was watching a YouTube earlier today that demonstrated the "pocket approach", but that's not what I had. What I think you're talking about is aligning the
edge of the board ALONG the pins. What I did was to "take FlatCAM literally" and PUT HOLES IN THE BOARD FOR THE PINS. The problem with THAT approach is when you make the holes small enough to control the position of the board in both x and y, the board binds against the pins, hangs up, the pin placement warps the board, the pins stop the board from contacting the tape correctly and so forth. I'm really trying to understand this, it seems like the combination of "holes for the pins" PLUS using the clamps included with the kit instead of tape, THEN not having height correction PLUS apparently "soft" engraving tips (well I'll find out on that one soon enough) ALL TOGETHER basically made any kind of success impossible REGARDLESS OF HOW LOW I WAS WILLING TO DROP THE STANDARDS. Fair enough! Lesson(s) (almost) learned!
I "get" what you're saying (I think so anyway) about the KIND of alignment "strategy" that will work, there are a number of them that would be adequate. I wouldn't have trouble with aligning
along the edge of two pins, and maybe deliberately having a tiny drill hole at one side's origin, a "poor man's fiducial", to keep it simple. Right now my "spoilboard"is 1/4" masonite, I don't know how "unlevel" it might be, I'm certainly willing to go to maybe acrylic sheet which I have. I don't really have any fancy microscopes or anything, also I'm not selling these, they're just internal prototypes, I do have vias to put rivets in but if they're a little off-target no biggie. Now it DOES mean I'm counting to a certain extent on the board outline being pretty reliably straight, maybe that's what I was unconsciously (and erroneously) worrying about. But in the end you're absolutely correct, the "dimension" that you need to worry about first, last and always is the ACTUAL Z location, ESPECIALLY because of the tiny height of 1 ounce copper.
Next to go in the trash is the Art3D tape. If you think I still missed or misunderstood anything, please speak up! (There's enough material JUST IN THIS THREAD to make a pretty decent tech manual, or a couple of chapters of one at least, if you're inclined to write it, go look at Amazon for material on the various things you can do with the ubiquitous 3018 mill, there really isn't much available, maybe "this generation watches YouTube but doesn't buy books" but I'm just not sure about that, with everyone making noises about STEM education and so forth...)