My machine only has one needle position. And my foot is wider than 1/4". So I determined where I needed to position my fabric that would result in blocks coming out the desired size. With practice I've managed to become fairly consistent. My seams are not always perfect, but more often than not they are straight and the finished block ends up the right size, so I'm happy with that.
Another trick I've learned is to cut just a bit differently, lining up the edge of the fabric so you can just barely see it on the "far" edge of the line for the measurement you want plus being sure the rotary cutter is held straight up, not angled toward the ruler. This gives you just that tiny fraction more space to maneuver when stitching and helps with making sure the pieces are cut consistently the size needed.
Hope this helps!