Do you have chickens right now? Do you have eggers?
Why have you picked Chanteclers as your meat bird? That's not a typical choice that I'm aware of, so I'm interested to hear your reasons.
If you want a heritage meat bird, I'd recommend the New Hampshire breed, particularly the strain bred by Freedom Ranger Hatchery. They are the best at putting on weight for a heritage breed that I know of. Some folks are also happy with Delawares, or some of the other Freedom Ranger hatchery offerings.
For cornish cross, the most popular meat bird, they have a feed to meat conversion ratio of 2:1 if I recall correctly. So 2 lb feed converts to 1 lb meat. These are the most efficient birds. Any other birds will need more food to gain 1 lb of meat. So when you look online, that will help you know what to do with the CX numbers. I've heard that heritage breeds can be around 3:1 to 5:1, but you might want to check that for the individual breeds of interest to you.
Any other breeds are not nearly as efficient as CX at gaining weight, and therefore will usually cost more to feed (feet cost can be reduced slightly by foraging, but even the best foragers can't substitute more than about 30% of their commercial diet with foraged food without seeing a loss in either health and/or productivity. Also you'll need an awesome foraging environment, which can be hard to provide anywhere in the world that doesn't support a feral chicken population.)
Heritage birds are typically slaughtered between 4-6 months, so you'd really need to know what their weight was at that time. Then if you can estimate their feed conversion ratio, you'll get an estimate of how much food they'll eat. The numbers you see online are mainly for live weight, when they tell you a rooster will be 9 lbs, that's 9 lbs live weight. That includes all the bits you won't eat.
Processed birds' carcass weight (only meat and bones and maybe skin left, the entrails, neck, and feet removed) is usually about 75% of the live weight of the birds (if I recall correctly) but can be 2/3, it really depends on the bird. Processed weight or carcass weight is the number to use when calculating your cost per pound of meat produced.
Best way to figure out exactly the food they're going to need is by experience for your particular strain of birds that you actually have. How you manage the birds, how much exercise they get, any predator or health issues, and how much they forage also play into cost per pound of meat produced.