The bonuses are added in two groups which are then multiplied. For example (not the actual numbers!): (resource items + relics) * (BH Bastion + Buildings) = (1 - 0.50 - 0.10) * (1 - 0.14 - 0.27) = 0.4 * 0.59 = 0.236 = 23.6% consumption.

Now, I don't know what are the actual groups, but I did manage to find an explanation for Sparta (which I don't play):

1. Temple of Demeter and Grain Reductions from the Academy are applied to Base Food Consumption. These percentage reductions are added together and applied as one reduction, rather than applied cumulatively in sequence.
   This creates Adjusted Grain Consumption.
   
2. The following reductions are then applied to Adjusted Grain Consumption: Grain Consumption Reduction Items, Consumption Reductions from high-Level Farms, Bonus from the Capital Knowledge and Grain Consumption Reduction from Hero Abilities.

I am unsure if all under 2 are added or accumulated (multiplied).

If anyone here is playing Sparta, please translate.

@Bank of ears, if you want, drop me a message in private and we can try to figure it out from your numbers.