Personally, I believe if you can offer the best, why not? I know some dogs can do OK on poor quality food, but the advantages to feeding a quality diet far outweigh the stress on your pocketbook...in fact, you feed less of a high quality food than you do the cheap stuff, so in reality it evens out...not to mention, less clean up (less poo produced)! Quality food is much easier on the system and the nutrients are absorbed and utilized better in the body. (less fillers...less waste) I used to feed Nutro Natural Choice and Diamond Lamb and Rice..my dogs did OK on them, but when I switched to Dick Van Patton...whew, what a differnce! Thier coats bot shinier, thier eyes brighter, less tears, more muscle tone (my 7 year old looks 2 again!) less gass, no oily skin...just a phenominal difference.
As far as how much to feed.... stick with the amount she is currently fed. If she seems to be gaining too much, cut back by 1/2-1 cup...looking thin..cut back by 1/2-1 cup. Every dog is different, just like people..some are more active and have faster metabolisms, so they need a bit more..others can gain weight from air! Let your dog's body be the guide. A run down on what my house AB's get fed...
Jasper (100 lb male) 2 1/2-3 cups daily
Chewy (70 lb female) 2 cups daily
Widow (70 lb female) 2-3 cups depending on work
I also supplement raw chicken and veggies a few times a week.
I feed once a day, in the evening for dogs over 1 yr of age.