I think I’ve finally got my OrdBot trammed. My bed had a pesky lean, which I finally tracked down to having precision washers between the eccentric spacers and the MakerSlide wheels on the Y carriage. In the process of tracking this error down, I trammed the X gantry to the Z columns, the Z columns to the base, and the X and Y axes to each other. They were all out by a miniscule amount (except for the bed).
Pro Tip: Use a 1-2-3 block to extend a parallel edge out to your square. Previously I’d judged the X gantry to Z column tram “by eye” with my square, which turns out wasn’t that accurate. After reading a forum posting where someone described how they trammed their ShapeOko using 1-2-3 blocks, I had a “slap-my-forehead, how obvious” moment and tramming became the OrdBot became much easier. I’m mentioning this here just in case someone else is at the same place on the learning curve as I.
Time to break out the shim stock and clamp on an indicator. I’m within 0.05mm front to back but it’s enough to cause prints to lift.
Pro Tip #2: before your ABS print detatchs, use some gel superglue to tack down one side to the bed. When the part cools down it’ll pop right off the bed and you can peel the glue off. Saved a couple of prints this way.
I used my dial indicator and some shim stock, and got ¾ of the corners level. One is 0.004" lower than the others. Unfortunately it’s floating because the bed uses the 3-point mount holes. I suppose I could spin the bed around and re-tram, but it’s good enough for me. I also took the opportunity to install an automated Z probe (and this time I got it working). I’ll have another post sharing how to do this, as configuring Marlin is tricky. I also turned on “bed levelling” to counteract my low corner. The difference is too small to be detected by the probe 
Since I have automated Z zeroing now I got rid of my ¼"-thick mirror that I clamped ontop of the heatbed and instead stuck down a PEI sheet. The bed heats up more quickly now (surprise surprise, no big heat sink on top!) and I’m getting the best prints I’ve ever had. OK, I’ve halfway through the first print but it’s looking really really good.