Not trying to be argumentative here... Ran across a paper on LMB feeding kinematics. They define gape as the vertical distance from the tip of lower jaw to tip of upper jaw... kinda on an angle with lower jaw further forward. They do have a plot of their gape vs length (Fig 5) They were primarily evaluating kinematics of the capture, not size capacity.
The Journal of Experimental Biology 198, 419�433 (1995)
http://jeb.biologists.org/content/198/2/419.full.pdf
