I think if I was you, I'd plan on finishing the bag of hills then swapping to something else - but do save about three nights worth of the hills so you can transition from one dog food to the other slowly. Is recommended to swap over 1/4 at a time eg first night 3/4 hills 1/4 the new stuff, next night - half and half, and the next night 1/4 hills and 3/4 new stuff, then the next night - all new stuff...
How much is the right amount depends on the dog, the dog's weight, how active the dog is, and whether it is desexed. So my dog is very active, but she's desexed. Desexed - you can cut back the food by 1/3 the recommended amount. To start with - I add the low and high end of the range of the amount to feed and divide by two, then I divide that by 2/3 - and that's what I'd feed. If I'm adding casserole, I halve again and feed half the daily dry ration plus a casserole ration. But it was hard to figure out how much casserole was appropriate to feed at all.
And on top of that I feed treats and kibble as treats when we're out walking and training so I figure that makes up for the extra required by an active dog. And I weigh her from time to time, and measure around her waist. If she was starting to get skinny, I'd increase the food. If she was starting to get fat, I'd decrease it.
You can also tell a bit by how fast the dog eats it. Ie if my dog leaves any food after 10 minutes, I pack that up, and feed that next time and reduce the total amount by the amount left over. If the food goes extremely quickly and she's losing weight, I up the total by 10% then - 10% again if no change after a week.
What ever the daily total works out at, split that by two for twice daily feeds.
Our Aussie terrier used to get half a cup measured, per day, of what we called "worms" which where like dry food only soft - like worms. And she'd get some table scraps. And if the painters were at our house - half of each painter's lunch - ie more than anyone single one of them got. It's those big brown soulful hungry eyes. Poor starving doggy.
A real lot - depends on the quality of the dog food. Some dog food is pretty close to sawdust - so you have to feed the dog more to keep condition on them.