I love the visual chart! :-)
Hi Ben, I have a gsd/husky mix as well. IF his metabolism inclines towards his husky parent, he will need/want less food than it says on the bag. Husky metabolism is very efficient...bred to be that way...i.e. able to run in front of a sled for miles, eat a frozen chunk of fish, then do it again the next day...pretty amazing.
Also it sounds as though your dog isn't running for miles every day...so he isn't burning a lot of energy. If he is acting healthy and active, I think he should be fine!
I don't know how much water is he drinking, but if you're worried, you could ask vet...he will drink more if he is fed dry kibble only, and if the weather is warm...but if it seems weirdly excessive, I would check his health.
My guy (75 lbs) eats daily: 1.5c. dogfood 2x a day + 1 dental biscuit + approx. 1/2c. training treats
Rumo ~ rescue shepherd/husky mix