Not sure why this aint posted before but last try.
Depend on the EM/EM pup.
At 6 months around 6 cups, 9 months to 18 months 6 to 9 cups. (dry food)
English Mastiffs vary greatly as the mature at greatly different rates per bloodlines. So reach 90% of the adult weight by 1 year and some reach 90% of there adult weight by 2 years. Full maturity is generally between 2.5 and 3 years, tho some lines take up to 3.5 years.
Best way to judge is 2 to 3 ribs easy felt with apply little or no pressure.
Stay away from chicken based foods as 75% of EM have allergies to processed chicken. Also they need grain free.
Most decent brands are 60 to 80 dollars for a 40lb bag.
When looking at food you should look for calcium and phosphorus ratio of 1:1.4 or lower for english mastiffs.
The standard for English Mastiff males is 160 to 220lbs so any EM that needs to be at 125lbs to be fit is either from a BYB/very poor bloodlines/ or way out of standard.
This years top EM at WM was around 230lbs and you could clearly see 3 ribs outlined. Comparing builds between a Great Danes and a English Mastiff is like comparing a Greyhound to a Basset Hound.
If you decide to get a pup, make sure to visit both parents, and view the breeds home and where they are raised, pups should never go to there forever home till 8 weeks some keep as long as 12, most 10.
Health testing is also very important, trust me the are prone to hip, elbow and eye issues. You may spend 1500 to 2500 for a parent tested pup, but it can save you 10 times that in vet bills.
If your rescuing research and research there are many issues you can tell by glance. Also if you rescue please remember it can take them 8 to 12 months to fully trust you.
Been around or owned English Mastiffs, proper name Mastiff, for around 25 years.
Best wishes, once you have a Mastiff in your life, you will always want one in it.