I do not use the lines on the cutting matt usually to line up my fabric. I use my ruler. I will straighten the edge of the fabric; I will then place a piece of tape or one of those little removable tape arrows pointing to the line I am going to use. So, to cut a 2-inch strip, I would place arrows pointing to that line, line the edge of my fabric up with that two-inch line and make my cut (most of your fabric will not be under the ruler, just the two inches). The extra half inch on your 2.5-inch ruler would not have fabric under it. This method really secures the fabric under the ruler and I get a good accurate cut. Also, I find I like to use a bit wider ruler as it is easier for me to stabilize the ruler with my hand while making the cut.