Welcome!!
I would cut two 3 1/2" blocks with your 1/4" seam (using a fabric that is similar in weight to what you will use in the quilt). Sew the blocks 1/4", press and measure the result. Do you have a block that is 3 1/2"x6 1/2"? If not, then either adjust your needle or mark the 1/4" spot with tape and try it again. This is slow but helpful figuring out where your 1/4" is.
I would also starch your fabric -- that will keep if from stretching (especially if you have to "reverse sew" -- something all of us do when we make mistakes).