I would use the two ruler method. That's how I was taught at the LQS.
I would use the 6 X 24 for the cutting edge and use a smaller one next to it and run it up and down the fabric to make sure the 6 X 24 was lined up properly.
If you wanted to, you could just use the 12 1/2 inch ruler and scoot it a couple of times. For me it would take positioning it 3 times because I would want to have plenty of cut edge to line up for a straight cut.