If your design has tracking that is resistance sensitive, you need to ensure your track width calculations are based on the minimum finished copper thickness your design can tolerate. We thought we had done this and then we found out the cost of copper had gone up. We had a batch of flexis made and everything was good. Next batch, oops. When the cost of copper rises, the PCB copper thickness supplied by the PCB material manufacturer can be to the bottom tolerance. Why? To save money. Cynical viewpoint, maybe, what do you think? If you base your calculations for 1 oz copper on 35 microns or even a little less, allowing for cleaning and etching you will be disappointed with the results. The finished copper thickness for our manufactured design was sub 30 microns! We now specify the minimum thickness of copper we require for the manufactured design. If your design has long tracks that are resistance sensitive, talk to your fabricator and enusre you know what the lowest copper thickness you might get. Once bitten twice shy.