I cut the thicker "grooved" Trex board down about 1/16" by setting the table blade height to "scary" and trying to avoid finger contact as I pushed the thicker boards through.
Just as a reference: Trex Select comes in 5.5" widths. I used seven boards at full width, and the end boards were cut to 3.5" wide. This seemed to work fine, but you may want to do your own measurement.
