The feed consumption is entirely related on how long you keep them. How long you keep them will determine their finished weight. If you 'forget' and keep them a week longer than you thought you would, you'll be rather surprised by how much food they'll gobble down during that extra time. It can really affect your economics.
Here is how to roughly estimate how much food they will eat.
a) The least efficient chickens possible, laying breeds, will convert 3 pounds of food into 1 pound of body weight. This is called a the "Food Conversion Ratio" or FCR.
b) The most efficient meat chickens, the super hybrid jumbo monster Cornish Cross, have a FCR around 2.2.
c) The freedom rangers are advertising FCR's around 2.5.
So, if you always estimate you'll need 3 lbs of food for each pound gained, then decide on how heavy (live weight) you want them to be, you'll know how much food to order.
Now don't look too deeply into a, b and c above... it would take most laying breeds an eterinity to reach broiler size (even with a FCR of 3:1, they won't eat nearly as quickly as a Cornish X). The same is true with Freedom Rangers. They have a very good FCR, but it takes them an extra 2-4 weeks longer to get to the same weight a Cornish X broiler would. But, I find it to be a very good trade-off.