I think it was mentioned earlier that your fold of the fabric should be at the bottom of your mat, close sort of to your belly. I usually fold my halved fabric another time, place the fold close, line up one of the lines with the bottom fold. It should run parallel to the fold, and then cut. We all seem to find many tips to help. The 1/4" cut check is a good tip.
I have also heard of folks getting PVC pipe cut long enough to bring your table up to the height you want. Put the legs(of the table) into the pipe, it should be stable and the right height for cutting.