I've had some PM's regarding a deer I shot some time back and here is the technique detailed. It is really never "unknown distance" if you keep a minimum of wits about you.
I use this mostly for my ELR shooting as I simply will not spend the money the more extreme lasers demand. If my Leica can't range it, I'm happy to hit on the second or third shot anyway.
http://www.youtube.com/watch?v=FeYgJRQG7ps