Thanks Philip. I’ll give it a try with penne, probably a bit cheaper than boxed mac.
Here’s what I’ve made so far:

2 boxes TJ’s Mac n Cheese
2 cans TJ’s Turkey Chili
3 cheddar sticks
Cook mac, drain, add powdered cheese, add chili, dehydrate.

*Dogs are great at helping with tasks that require zero dogs. Get a dog helper.
I split the batch into three portions.
~750 calories each, 5.65oz dry. Add one ounce and 100 calories for a stick of cheddar melted on top. It’s also at the upper end of what I can eat in a sitting (which is good). I also top with red pepper flakes and salt.
So this started out at a bit over~19 oz wet, down to 5.65 dry. By weight, that’s roughly 14 oz of water lost.
The volume is a bit over 1.5 cups dry…which also happens to be pretty close to 14oz water by volume.
The ~1:1 ratio by volume seems to hold true.
Will cook up a batch later and update how much water/time is needed for rehydration. Based on above, will start with a bit over 1.5 cups water for rehydration.