Free: Contests & Raffles.
I just go with my gut reaction. I've been within a 1/2 inch or off by 20 inches...it's hard to do consistently well, especially with photos. Mike Eastman has a great method for mulies that he's perfected over many years. When I try to add up lengths and mass guesses, I'm usually way off. I'm better at sizing up a "frame" and giving it about 10 seconds of thought. I may be off on this bull, I'm just guessing. It's easy to over judge elk. A 300" bull is a very nice mature animal. If you can get photos of a net ~300" bull it will help you ingrain that image for the field.