Benjie,
You can't measure the AMO lenght of a bow. The AMO length is 3" longer than the string that properly braces the bow. That's the definition of AMO length. The response above is the proper way to measure a bow, but the number you get is the length of the bow, NOT the AMO length of the bow.
Also, as Old York says, brace height is properly measured from the deepest part of the riser/grip/handle to the string at 90 degrees.
Take a look at the AMO Standards Book in the How-to section of TG and it will explain all this stuff. What it doesn't explain is why they came up with the term "AMO Length" and defined it as they did. All it does is confuse people!!!!! It would seem to me that the number people need is the length of the proper bow string for their bow. A better definition of "that term mentioned above" (to me) would be a number that equals the length of the string, period. No need to confuse the issue by 3 inches in my opinion. BUT THEY DID.