Finding your correct footwear measurement

My foot measures 10.5 inches long, and I usually wear a size 10.5 shoe, though sometimes a size 9 works too. The rule of thumb is that women’s shoes are about two sizes larger than men’s for the same fit. That part confuses me because my mom wears an 8, but those don’t work on me. Meanwhile, I wear my girlfriend’s high heels comfortably; they’re roughly the same size as mine and fit almost perfectly. They feel a little snug, likely due to all my weight pressing down on my toes. So, how do you actually determine your correct size?