Ballet SLIPPER SIZING?

Ballet flats sizing really depends on the manufacturer. I a guy and I wear a guy's 8 1/2m That makes me a women's 10m for most ballet flats. But lately I've been buy Michael Kors flats and I need an 11m in them. And to really mess things up my Puma ballet flats I need a 12m. Best thing to do is find a store and try some on