I did this the same way -- mark the date on a new bag upon opening and then note the date when it empty.

We feed kibbles + canned, but the math for that was easy since each dog gets a can a day (1 can split in half, morning and night).

For us, with two large dogs, that means 30 lbs bag = 20 days, in which time they will need 40 cans of food.

Our approach to this is simply to buy extra food and perform stock rotation.

I tend to shoot for 30 days, so I have 9 cases of canned food in the garage, and three bags of dry food in the house (don't want it to collect moisture) in addition to what is in the pantry.

As soon as we take food from those stockpiles we replace it.

Also as suggested above, I compute the water usage the same way we do for humans.

In terms of first aid, I make sure the normal kits cover the dogs as well. For first aid, I tend to use a decentralized approach, with a fairly full kit in numerous locations which allows a kit to always be close and increases the amount of supplies overall.

-john