I based my design on two pieces of ply making up my cross members. I assume that 18mm thick might indeed be 18 mm so two pieces joined would be 1.416". In reality they are averaging 0.689" thick which is 1.38" with two joined face to face. So I am 0.007" off. I made my SketchUp drawings based on the 1.416" and I ran a cutlist (not SketchUp's cutlist) using those numbers. I dont want to pre-cut and come up short. Is that a need to rerun SketchUp with the exact numbers for thicknesses?