Just replaced coolant and flushed the radiator with water over the weekend. I can only refill about 8 quarts of 50/50 water/anti-freeze mixture into the system. As per owner's manual, the capacity should be 10.6 quarts. Assuming there is 2.6 quarts water in the system before refilling (because I flushed the system with water), this makes the overall water/anti-freeze mixture 62/38. How do you guys deal with this problem?

I just had the exact same experience on my "95 C280. To try to correct it I drained about a quart of coolant mixture out and replaced it with a quart of 100% coolant. This will get you close to the 50/50 mixture. Actually 60/40 is ok. It will actually transfer more heat at 60/40 than at 50/50, but the freeze point will be slightly higher and the boiling point will be slightly lower. I would still shoot for the recommended 50/50 to ensure that the anti-corrosives are right.