I understand the maths but consider this...

An economy flight to Hong Kong (using January fares) is £684.19. Of this, only £320 is the fare. 12.5% of this is £40 which makes the new fare £724.19. This is about a 5% increase. Surely most would pay that for 8 across rather than 9. And even if you add in the fuel supplement it still stays below 10%. And then consider the weight saved (1 person+bags * 16 rows in economy) meaning additional freight can be carried (which could still further reduce the BA fare element). And increased comfort levels for about 150 passengers. And you would be matching the competition.

So why have BA chosen 9. Did they learn nothing from the 777 10 across fiasco?