Well, there's part of my problem...
My "thin" 3-ply material is 0.090" thick, which is thicker than the 3-ply on my Thunderbird, which is only about 0.062".
Thicker material means the 45-degree bevels and countersink radii are wider, on the top surface, by the same amount as the difference in height.
Which will put the TOP of the bevel about 0.06" closer to the top edge of the countersunk hole. 1/16," essentially.
Which was right about the offset I was aiming for; around 3mm screw edge to bevel; which is what my actual T-bird sports.
So I suspect I either need thinner material, or I have to start over; my screws will be right at the edge of the bevel... unless I go to smaller (like #2) screws as used on the truss cover instead of #3's, or don't countersink full thickness. Or both. That'd buy me a few more hundredths. But would probably bother me.
I never noticed the difference in thickness on the pickguard, even though I recognized the single-ply truss cover and control cover were thinner than "typical."
Drag. Lost time more than anything.