Kelly: I was referring to the diameter of the vacuum pipe. I first used 1 (or maybe 1 1/2") but went to 2" when I was making some other changes to my table and the increase in holding force was dramatic. My table base is a couple layers of 3/4 ply with one side laminated with HPL and edges sealed.
Then the vac plenum out of 3/4" MDF (8 2x2 zones) followed by the spoilboard. If I'm carving a small piece often I will cut a mask out of some scrap and hold down with vacuum or clamps on the mask but most of our work is panel processing so T-tracks would be wasted.