A late entry on this thread, I know ...
But I want to second what Steve said about throwing your ruler away. Measuring is important for some things, but I think it can slow down your learning process. Certainly you should start by reading what characters and ratios are important to look at, but once you know which points to key in on, then spend your time browsing pictures of beaks and the beaks that you have.
What helped me the most was looking at a large number of beaks and developing an intuitive sense for them. Your eyes and your brain are very good at perceiving proportions and integrating visual information that's just too complex to be described in ratios, and the faster you can train them the easier your job will be. You want to be able to look at a beak and your mind automatically go through a check list (jaw angle, hood-edge ratio, lateral wall features, wing fold, hood-crest gap, overall "posture", etc...) and family name or two pop into your head. I think the fastest way to get there is to look at as many beaks as possible as often as possible. The information seems to leak from my brain rapidly, so I try not to let more than 4-5 days go by without looking at beaks from some source.
Then measure if you must.
Just my 2 cents ... good luck to you!
Gayla