There are (some) good answers in this thread already. I like the above as a simple eyeball approach.

Other considerations: will you be changing out tires for different uses or seasons? The stays are adjustable but don't trim them down for a narrower tire and then realize you will run out of adjustment room once you put your studded tires on (for example).

One good recommendation I've seen is to keep the back edge of the fender tighter to the tire, to help keep objects out, and the front edge somewhat more open to help release the crud that does end up in there.