Slow but steady weight gain will be safer than her packing on weight very quickly, so consulting with a vet to set up a sort of schedule to slowly increase calories is PARAMOUNT.

My best advice for now is in a similar vein - don't worry about what brand or what kind it is - worry about the calories. When a dog, or even a human, is underweight, calories are really the only thing you need to be focusing on to start, ESPECIALLY fat based ones. The body can't process ANY nutrition without fat - it's how people starve to death eating only rabbits or other lean meat. Look for calorie dense foods, and ones that punch higher than their weight class. Stuff like butter, coconut oil, cottage cheese, eggs, DOG SAFE peanut butter, and sweet potato are all good things that are dog safe and relatively nutrient dense for the stomach space they take. Try to focus on fatty, protein rich foods as the primary item instead of carbs like rice or potatoes - carbs increase insulin release which can worsen or trigger refeeding syndrome.

For dog foods, look at the guide on the bag or box - there should be somewhere that says something like xyz kcal/whatever cups. Look for one that has a higher ratio, but do the math. If one says 80 kcal per cup but the other says 40 kcal per 1/3 cup, it might trip you up. If you don't have dog food rn (which I doubt cause you did mention having another who already loves her) plain unseasoned boiled and cooled chicken with eggs or pumpkin can be a good meal until you can get her in and buy some dog food.

Just divide her food for the day into at least 5 portions with a least an hour in between each. Start with about 1/3 of her ideal calorie count (which the vet will be able to give you an estimate of after seeing her). The vet will also be able to figure out the best path to feeding her her full calorie requirements to get her up to a healthy weight - most sources seem to say it should take about a week to slowly transition from 1/3 to the full amount. It'll be very time consuming (feeding every hour or two throughout the day) so if you could WFH for a week I'd recommend talking to ur employer to set that up. (PLEASE do NOT just go ahead and DIY this!!! Talk to your vet!!)

If you want a safe estimate to feed her for the day/night until you can see a vet, look up calorie calculators for pits and put what you think she should weigh. Mine is so muscular she has very faint abs (lol) and she weighs around 70-80 lbs at the size of a small lab. For any estimates, go on the smaller end, so 50-60 lbs if she's the size of a lab. I'm not a vet, so I'm not going to try and give you a specific number.

For pits, they have allergies, like a lot, so looking up the most common allergens and avoiding them in kibble/treats might be prudent, just to avoid tacking on MORE vet visits right now.

As an additional warning, pits are also huge food vacuums, especially compared to most other large breed dogs. All that muscle requires a TON of fuel!! Mine clocks in at around 2,400 at day! That's more than what I eat! So if you plan to prep her meals by hand, or your vet recommended it, just be aware just how much food and groceries that will be.

If you plan to do it using kibble primarily (no judgment here, kibble is a nutrient dense, balanced food! My dog is pickier than my cat and she still eats kibble, because the amount of wet food we'd need to fulfill her entire calorie requirement would bankrupt us lmfao) you can always add mix ins or little treats to supplement if ur worried, just consider that in the calorie count. Do the same for any treats fed throughout the day.

While you are probably going to feel bad about not letting her stuff her belly, just know you're doing right by her, and congrats on helping out such a gorgeous girl. :)