Today, I discovered something that is probably pretty obvious to most, but still might help others like me. I use Mdf board as a sacrifice sheet, and level when I install it. Then one piece will last me for a bunch (20 - 30?) of cuts. However, I now realize that over time, its surface warps – not enough to be visually apparent, but enough to cause a 0.5 - 0.8 mm differential between high and low spots, which is enough to cause problems when cutting PCB’s. (I had thought the highs and lows were the result of cheap PCB board.) Since I like to cut with a 0.6mm drill at 0.5mm depth, multiple passes are required. Today, I figured out that if I level the Mdf immediately before cutting, one pass is all it takes. Beyond this difficulty, I am able to produce nice, double sided, fairly large (about 4" x 6") PC boards. TinyG, ChilliPeppr and EagleCad are really great.
Edit – I meant 0.05 - 0.08 mm height differential, and 0.05 depth of cut.