I wouldn't worry as much with an insulated boot. You are right that sizing does vary slightly from manufacturer to manufacturer, but insulated boots will never be as precise of a fit as other types of footwear. Based on reviews, I went up half a size in the pull on version of these boots and found that they're a good fit with thick winter socks. That said, I probably could have gone with my regular size. If you sometimes have to size up, then I would size up in this boot just to be safe.As for a direct answer to your question of how sizes are calculated, each manufacturer calculates them slightly differently. If it's a real concern that you'll get the wrong size, then try to find them locally. Or you could order from Zappos since they pay for shipping on returns. However, they only take returns for unworn shoes. Backcountry has a much more liberal return policy.