What I do is tape a few toothpicks vertically along the center line of the table. Then I fold the backing in half and put single basting stitches in an X at 12" intervals along the fold. I can get by with not pressing the fold into the backing that way. I line up the X's over the toothpick center line, and clamp the backing to the table. I double-check that I'm in the middle by measuring. After the batting and top are on, I can usually figure from the design where the middle is, but i can also feel the toothpicks, too. Of course, my table is sort of narrow, which probably makes a difference.
Charlotte, hoping that makes sense