I always use two rulers when cutting my fabrics. They are not special rulers just regular ones.
I will try and explain without pictures. I am righthanded so I have my fabric to the left of me and I line up the first ruler the length of the fabric at the width that I want then I get my second ruler and run it down the whole length to check that the first ruler is in place. Sometimes I need to adjust the first ruler a bit to the right or the left to make sure that I have the accurate measurement. When happy I remove the second ruler and then cut.
I hope this helps.
P.S. I just found a photo that my help explain a bit better. You can see how I have placed the rulers on the fabric. When looking at the photo it is the bottom ruler that I remove.
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Last edited by busy fingers; 05-17-2016 at 01:11 PM.