Thank you, Gerry. Cloud lifted. Your statement, "[a]s you reduce the stepover, the ridges get smaller, but only because subsequent passes are removing the high spots left from the previous pass" really brought it home for me. As a threshold matter, my measurement of the amount of deviation was flawed. I failed to account for stepover and the fact that subsequent passes would be removing high spots. So, at the bottom line, I really don't know the amount of deviation, and without having that number, I can't calculate the deviation angle. That also means I have no idea how much I need to shim.
The biggest problem is trying to calculate the deviation angle. I suppose I could make one pass at a depth sufficient to ensure full bit contact, measure the difference in the DOC at each end with a dial indicator, and use trig to get to the deviation angle. From that, I can use trig to calculate how much I need to shim to correct the deviation at the gantry level. Another option would be to level my glass plate on the spoilboard, and use my dual indicator tramming gauge to measure the height deviation. I'm thinking the tramming gauge should yield more accurate measurement. Trig does the rest. I'm not comfortable just laying the glass on the spoilboard, because there is twist involved and it will probably result in an inaccurate measurement without leveling.
You may be correct about not seeing the error in normal cutting. I'll have to crunch the numbers and see how it works out with the right ones. I am now confident that the deviation is more than .001", since I was reducing the true deviation (cutting off the high spots) with each successive surfacing pass. I'm pretty sure the deviation is at or less than .005", since I had full contact at .005" DOC. but if it's as much as .005, that's 5X more than I originally thought it was.
Does the methodology outlined above make sense, or am I still missing something?
Thank you so much for getting me on the right track.
Gary