I think the OP is using "X" and "B" as variables to represent destinations, not as fare classes.

He just wants to know if he can predict which flights will have empty rows of seats in coach. I don't know of any site that does this.

OP -- Either way, this is a fool's errand. Even if you did manage to find a flight that consistently was 65% full in the back, the odds of finding a flight that's 65% full in the back where NOBODY else on the flight takes any of the seats in the empty rows are just next to nothing. It's a matter of luck, and the empty rows usually go to the status passengers.

Better solution: earn a bunch of miles (through flying, or credit cards, or any means necessary) and use those miles to upgrade yourself to business.

And Welcome to FT!

BTW, regardless, I applaud the analytical approach. If you're a software engineer in the Bay Area please PM me. :-)