When using my planing sled to thickness thin parts, I use a simple trick to accurately mill and measure my pieces. I use double-sided tape to stick them down to a backerboard. To get reading of my planing progress, I cut a notch in the backerboard. This way you can slip the nose of a dial caliper into the notch and measure the thickness of the workpiece, getting an accurate measurement without removing the workpiece from the sled. Double-sided tape is applied to the backerboard to hold the workpiece down and allow it to be planed. The extra thickness of the backerboard carries the stock through the planer and lets you plane it to any thickness. A notch in the backer is sized to fit the nose of a caliper. You can then check the thickness without removing the workpiece.