Trimming a large quilt is a task. I used the long straight ruler to start one side, then took the perpendicular side off of that line. My large aluminum T-square is sold for drywall work, but I'm combining it with the straight to get a 90 degree edge. Also using the table edge to set the T-square.
A big 48" T-square that Home Depot markets as a drywall measuring tool is my go-to for squaring up my finished quilt. I use the "T" end against the edge of the table to determine that 90 degree cut, then move to the other edge and continue. (Sorry my photo clipped the T-square.)
