Re. the video, not sure why she uses a second ruler for measuring the strips when the fabric is sitting on top of a gridded cutting mat. Just line up the Slidelock with the lines on your mat. Easy peasy!
SandyOHmm... my huge Sew/Fit mat is 15 years old and I can still see the lines just fine. I've rotated it every now and then to get equal wear at each corner. I'd replace it if I couldn't use the lines, certainly not going to use an extra ruler to do the job the mat's supposed to do.
QuiltyI've been sewing for 40+ years. One of the basic skills I learned from the beginning is to compare ALL your measuring tools to ensure they're the same - that means rulers, tapes, hem gauges and, as they became available, mats and rotary rulers. Of course, I'm from a farming family that built our own barns and did a lot of woodworking, so that's probably skill many people aren't taught today. I don't use inaccurate tools.
LeapIntoTheWildI think the most important thing is that you use the same cutting method all the way through a project. Even rulers from different manufacturers have different sized lines, and you could be off a little, which over several seams, can become a lot!