They are all public info, this isn't military. Go search it up, all the calculation are done via manufacturer's claim of standard two class layout arrangement, max fuel and range(Hence "stock" configuration from the manufacturer, I didn't specifically choose any airline's own arrangement since as you said each airline has different seating configs). Also note that I didn't comment on anything such as maintenance cost, availability but only per seat fuel efficiency(Not even CASK as that would require accounting all the other cost related) which could be calculated via public specifications.
Per seat fuel efficiency is calculatable because we know the fact that aircraft manufacturers calculate their aircraft's maximum range via the same standard(Advertised range usually is the range with max allowable fuel and "typical" amount of passengers aka the amount of seat the manufacturer advertises for two class config with basic reserves and ISA, no headwind) and we also have each aircraft's maximum allowable fuel and how many passengers the aircraft could accommodate in a "typical configuration".
But I after rechecking everything, I did realise I made a mistake. B737MAX-7's brochure actually mentioned the seating config used for typical range calculations was actually 138 seats instead of 156 based on this document I dug out:
Hence C919 actually isn't that much further behind since per seat at maximum range is only 6 percent less than 737MAX-7 albeit still with LEAP-1Bs, but I guess it ain't bad especially with that new composite wing coming online soon hopefully. But the part about C919 still behind the MC-21-300 are still valid, well atleast with their original PW-1000Gs. So, I'll apologize here for not double-checking data and being a bit overconfident.
