You missed my point. If you are getting 16s from your 35d (you say 16.8 now) there's no way that you get 20 (as you originally suggested) from the 50i unless, as I said, there's something wrong with your 35d. Others may get 20 from their 50i but then most people get 10 to 12s (or better) from their 35ds. If you could match the consumption of your 35d with others, then what others say about the 50i will be a guide, otherwise it won't be of any help ... unless your 35d is sick.
Even the ADR figures for the 50i see it use 67% more fuel than the 40d and, while the ADR figures aren't an accurate guide as to real-world consumption, their relativity to one another is accurate given the nature of the test procedure. Assuming the 40d and 35d are much of a muchness (40d is actually claimed to be a little better), then extrapolating your 16.8 to the 50i (using the ADR figures for relativity) you will get 27.9. Hence my comment that consumption you'll get out of the 50i (if everything is okay with your 35d) will "close to 30".