This might sound like a stupid (and perhaps obvious) question but... how do you tell when the shoe fits right? I have found that common advice is about a "thumb" in the front of the shoe, but is that a thumb width in the middle of your toe section, or would it be a thumb width from your longest toe? If that makes sense. Because a thumb width at the front of the longest toe would be more like 1.5 or 2 thumb widths towards the middle or opposite end of your toes. Obviously I am over analyzing a bit but I'm trying to pick out the right size and I'm not sure which will provide the most comfortable fit. This is for casual/boat style shoes, so nothing athletic.