I just bought a seam guide at the West Palm Quilt show. It's really different. It looks like a ruler, but is made of white rubber and sticks to your mACHINE or your extention table. You get a guide with a hole that the needle goes in, and that has all the markings on it. When you find the one you want, line up your "ruler" and away you go- it doesn't move for anything and is long. You can get different sizes, and the guage has measurments for 1/8, scant 1/4, 1/4 and then up to 1 1/2. of course you can move it out further, too.
This couple designed it and also sell the extension tables for all models up to a size 24 x 32!!! The prices are way better than dealers, so I ordered one, too. Here's the link:
www.sewverysmooth.com
It may be a little pricy for the ruler one time, but no gook and it will last forever- just wash the lint off.