It isn't about the seam allowance...it is all about the correct measurement of a finished block. Take 3 1.5" strips and sew together using your current method of getting the 1/4" seam. Then measure from the finished side of the middle strip, seam to seam. If you are 1" on the nose, you are doing it correctly. If you are off either way, then you adjust your seam allowance until you get the correct size needed. Or it may be a correction as simple as thread size or needle positioning. And when you change fabric thicknesses, you will need to check again. There are lots of variables and adjustments.