I just checked 170t gross weight, FL310 versus FL370, Mach 0.8.
Difference in fuel flow is 16.9%, so in line with the numbers I previously provided.
The slight difference is due to the different ground speed at constant Mach, as lower altitude means higher ground speed (this note is aimed at the people without ATPL - I don't even have a PPL).
We must consider that the Quick Check table is a bit unrealistic as 170t Gross Weight (reference for Quick Check I used) for an A330-300 is pretty low, actually only possible on a ferry flight or with very low payload.