Originally Posted by nycbikes

When it comes to sizing ctt and ctc are irrelevant except in knowing whether or not you are going to bump your privates. Since seatposts are thick enough and strong enough now to extend 300mm and more above the seatube, the best measurement for how the bike will feel is the virtual top tube. You can raise and lower your seatube dramatically, you can even leave a threadless stem long and use spacers to add length to a head tube, but you are limited to changing the length of the cockpit by two small means, stem length or seat position, and in changing either you are signifcantly changing other parts of the ride.