Guidelines4Quilting has a Prep Tool that gives you a scant 1/4". I've had it a couple of years now. You can buy their Guides, which are sticky tabs that adhere to your sewing machine and leave no sticky residue but are super thick and come (I think) 5 to a pack. I have been sewing scant 1/4" seams for so long and am finally making blocks that come out the correct size. Just go to Guidelines4Quilting.com and look up prep tool. It's good for other uses, also.