Welcome to the forum. I am glad that you have taken a few moments to address the problem your dog is having and the possible use of cranberry extract. My experience with cranberry extract and dogs is extremely limited, but we use cranberry extract in the house all the time. I use it with the cats from time to time to help alleviate UTI infection, and we also take it ourselves for the same reason.
For a dog of 35 pounds I would be using the child's dose which appears to be half of an adult dose. Basically you can't overdose on cranberry extract that is basically free from a lot of additives. I use a product by Natural Factors called Cran Rich Super Strength Cranberry Concentrate which I have found to be good for animals due to the fact that it has a very mellow flavor and is easily mixed into the food to mask its taste, and make it more palatable. I have tried other brands and found them to be stronger tasting, and less appetizing for the animal. Take your daily dose and split it in half giving one half in the morning feeding and the other half about 12 hours later in a meal.
I am enclosing a website that has an article on cranberry that is well worth reading, and will give you some insight on how it works. You can't go too far wrong with the cranberry extract. From my experience it seems to work better as a preventative than a cure. So follow through with your antibiotics as prescribed, and back it up with cranberry extract as the preventative in the future.
When it comes to supplements do your homework first before administering them. Some supplements can be extremely hazardous to your animals health if not administered properly. Even lethal.
Hope this helps. Dr. Lee has it bang on.