Couple of comments. If your mix is 20 pounds over weight, then obviously he is he is getting too many calories that he is using in a day. Every dog food bag should come with a set of feeding guidelines specific to that dog food. These guidelines are usually based on the weight of your dog (ie: one cup for every 50 pounds of dog, twice daily), and are a rough estimate of how much your dog needs to meet his nutritional needs.
We also go by activity, condition and how the dog "handles" an amount.
I'm going to guess your dogs are about 100 pounds, so that means ONE cup in the AM and PM and I would stop the wet food. You can use a little shredded cheese, or garlic powder etc to spice up the dry if they are picky. So Yes, you are over feeding.
Labs have a tendency to get overweight so I would begin now with the right amounts and exercise where possible.
If your mix is showing signs of pain and of excessive drinking, then he needs a complete physical exam to rule out kidney problems or diabetes. Also if he is developing arthritis (very common in dogs) he will need to be on a good non-steroid anti-inflammatory drug that will allow him to move more normally and perhaps get some much needed excersize.
Finally a comment about your food choice. I know money is fixed and you need to do the most economical thing. However, buying what is on sale and especially the low end Purina brands is, in my opinion, not best for your dogs. In the long run, if you purchase a higher quality food you will be facing less disease in the future and have overall healthier dogs. In addition, now you will be feeding almost half of what you are now. So please try to find a pet store that will consult with you about Iams or Nutro or another super premium brand. Also, your veterinarian may sell Science Diet which is also a very good food, but sometimes has problems with taste.
You can change dog food brands and diets. Many dogs do not like eating the same thing for years. Just do so with high quality foods, and change slowly. (over a few days so as to minimize digestive upset). I do this pretty often and have never had a problem.
I would think that you can feed your crew for $75 per month using a high quality food, less of it and even get your shep/bordr collie mix an exam and medications. Most veterinarians are willing to work with good clients for the sake of their furry family members.
i use whatever kind of Purina dogfood is on sale. there is 4 or 5 kinds and if i dont switch kinds they leave it in the bowl. my ex always said that if they were hungry they will eat. i say "I will not make an animal eat the same food every single day for 15 years and hate it" it just seems cruel to me. I know that they become increasingly picky and want at least a tablespoon of wet ontop of dry or they wont eat that now. have i created monsters? the cats are starting to play the same game! im on social security and end up paying $75mo to feed 4 animals. if they would eat less of a more expensive brand it would be worth the change.....as long as they dont stick their noses up. lol thanks sp