as someone said its a thread or 2 width less than a 1/4 inch.
i prefer to put those 2 threads into my rotary cutting instead of worrying about sewing just shy.
the 1/4 inch foot that came with my machine is perfectly a 1/4 inch so why mess with perfection when i can just add those threads into my cutting and sew the 1/4 inch.
so if you want to cut a 2.5 inch strip of fabric cut it at 2.5 inches and 2 threads.