Do you put on your interfacing on first extra large and then trim it down from there? In my case if I want a 10.5" cut square, I cut the interfacing at 12" and the fabric is still overlapping that.
I have a large variety of rulers to help me, but just last weekend at the goodwill I found a EM (easy mark) Grid package for cheap just in case I ever make another t-shirt quilt.
I do quite a bit of general fussy cutting and buy the large pieces of clear grided template plastic to use as templates/positioning. Be warned in advance, it is too thin to use as a ruler guide! Place the template, then your ruler along the template edge, remove the template to cut.