As John states, the measurements are to the center of the legs - this is also the center of the I-beams running between the legs. I choose to use the stated measurements (in my case, that was 30” between the centers of the legs/I-beams). I used 1/2” homosote on top of the plywood, so the resulting edge sandwich of plywood and homosote was a tad bit unsightly, so I used the addition space on the legs/I-beams to add some edge banding to coverup the edges (I assume you could also do this if you ran the plywood to the outer edge of the legs).
