Still can't find a free online copy of the seminal paper by Johnson & Post (Transactions of American Fish Society 1996) that started this...

Another paper with gape info for LMB. Their point is the pharyngeal gape is smaller and they think that limits prey capture.
http://fishlab.ucdavis.edu/W%26REBF95.pdf
When you do gape/length, they run .18-.19 around 230mm length, 9". They say these data are for FL LMB.