Poor.
EK works out what you would need to eat in the hotel, then negotiates discounts or typically, fixed rates for breakfast, and then reduces the allowance by the percentage of the discount.
The amount varies on your arrival time as dinner is more than breakfast. Approx values for LHR are £47 (early arrival - b'fast, lunch, dinner, overnight and b'fast) to £65 (late arrival with 2 dinner times).
Don't plan on saving anything from the allowances!