when I first started quilting I measured the fabric I wanted to cut from the left hand side of the fabric. I am right handed. My first cut was started against the ruler then veered off the ruler and ruined almost half of the fabric I had. Obviously there was a problem. After that fiasco I turned my fabric around....cut from the right side of the fabric. After squaring the end of the fabric, I measure over the amount I want to cut. Eg. a 3.5" strip. I use the ruler to measure then put the slide loc next to the ruler....remove ruler and cut. Works GREAT!! Before the side loc I had always used 2 rulers......just worked for me.
I would not feel safe putting the slide-loc on top of another ruler.