Paul, even if there was such a site it still wouldn't guarantee anything. A flight can be showing a 75% load when a few hours prior to departure it fills up with pax either from the airline's own earlier cancelled flight or that from another carrier. At major hubs there are always pax that have missed their earlier connection and are now booked on your flight. It can of course happen the other way around in that a full flight can end up with empty seats due to missed connections. These are only just a few example of numerous permutations. Then there are seasonal and daily flows in one direction or the other. You can try and go for off peak flights but even that is no guarantee. A large group might be booked on what would normally be a lighter flight. It's a lottery!