Don't mix your units.
When I first started in the PCB fabrication industry, I was fresh from college and ready to jump in and make an impact. The first hurdle I hit was likely the same as many first-time designers: units of measure for PCB design. Remember back in school, your physics professors or science teachers likely pointed out missing units in tables, graphs or homework assignments? That problem.
Figure 1. The three units of measure commonly used to describe the relative thickness of a PCB layer.