Qtools Sewing Edge sells a repositionable vinyl stop which would work well for this and on anything else, like a ruler, where you want to make a particular size. It is thick enough that you can butt fabric up against it. You can get them at www.aliciacrafts.com (no affiliation to me). I just love them.
I am making an OBW and I marked the 60 degree triangle ruler for 3 3/4 inches with it and wow did I fly with that! It made it so easy to get the measurement quickly and then rotary cut each time on the strips.
Originally Posted by Jan in VA
Here's a trick I've used for years to create perfect 1/4" seams and I sew strips together at warp speed using it!
IF your machine's feed dogs are NOT wider than the regular presser foot you use ----
1. Measure 1/4" to the right of needle strike (Use a piece of graph paper to help determine whether your needle-to-edge-of-foot is 1/4" wide).
2. Line up the glued edge of a small Post-It note (I use the 1.5" x 2" size), sticky side down, parallel to the edge of your presser foot at that 1/4" seam mark. (Like the tape does in other ideas posted.)
3. Use a fat rubber band from celery or broccoli stalks to wrap around your free-arm of the machine to hold the Post-IT in place. (I've found that the metal of any machine's throat plate will NOT allow the Post-IT to stick more than a few seconds.) This rubber band can usually be easily moved out of the way for refilling bobbins or making other than 1/4" seams.
4. A long strip of Scotch tape will also hold down the Post-It if you don't have a rubber band.
5. Align the edges of your strips/blocks with the edge of the Post-It and keep your eye on the front edge of it while piecing, instead of eyeballing the edge of the presser foot or the needle.
It's like this.......When we "drive" our machines with our eyes on the needle or the edge of the presser foot, we are driving like a 90 year old lady who is looking just off the hood (front) of her car to steer...she's got to make lots of tiny adjustments to stay reasonably straight. Looking ahead down the highway (or forward of the needle) makes for much easier driving/sewing corrections.
Jan in VA