Just remember dogs aren't built to carry a lot of weight.
-john
Good point, most healthy conditioned dogs can safely carry 10% of their body weight, equally divided on each side of the pack. As Susan mentioned weight is easier carried more up over the dogs front assembly, especially on many of the non-working breeds who are not square, meaning the same length as height. I’m partial to a good harness system under any pack to prevent it from creeping up on the dog’s neck. I do like the ruff wear harness for that reason.
I do weigh what I put in each side of the packs on my dogs with a scale and use Nalgene plastic water bottles with marks so if we stop for water along the trail it’s easy to re-adjust the water.
I own a square working breed that have good hip structure, so they can safely carry 10% of body weight and actually more than that, however, I've never asked that of them. Some breeds are better equipped for either weight pulling or carrying than others.