How you measure your fabrics while rotary cutting can also make a difference between needing a scant or 1/4" seam. If I use the far side of my ruler lines I can usually go with a 1/4", if I use the top edge of the line, I mostly have to use a scant 1/4" :wink:
Also the position of your needle in regard to your presser foot can be a variable in the seam width. As can how you see your fabric moving under the presser foot. One of my friends always has a thread outside of the presser foot while she sews...she says that she can't "see" under the foot LOL but she does have perfect seams :D:D:D
I make one block and measure it as I go, adjust my needle position, my cutting, to get the end result that I am looking for :wink: then I cut out the rest of my fabrics and start sewing them.