
If it's any help, I have a 2yr old doberman male that weighs 28.75kg. He is also on the Burns adult maintenance, and he gets 1 + 3/4 cups morning and night. As you have a lab (bigger build than dobes, and my boy is dinky for his breed too), I would think around 2 cups twice a day or a little more would be close to what she needs - obviously talk to the vet of course, just my thoughts on this. I also have a 21 month rott X bitch that weighs 28 kg, she is on 2 cups twice daily. She is also a LOT more active than the dobe - remember to factor in activity and breed when you're looking at amounts to feed (labs are prone to weight gain). oh, and treats, of course :)
Also, trial and error - it's taken me this long to get the amounts right for these two, the dobe was 6 pounds overweight until recently!
See what the vet says, hopefully now your girl is in your care she should go down (if she's over). and you aren't alone - I had a customer a couple weeks back (I am a groomer) with a severely obese cavalier - turns out she'd been posted elsewhere in her job for a few months and the pet sitter had fed him on nothing but bonios!