Road bike recommendations

OK. B2 + 14-16mm

If you draw a horizontal line from the red dot that's at the top of the head tube, back to the centre of the seat tube, that would be the effective top tube length. (Note this is the "little bit" more than B2. That measure is a fixed property of the frame geo (assuming your front and rear wheels are the same size!), so independent of seat set-up, etc.

It is'nt truly independant of seat set-up though. This is the problem, and why Tester mentioned Reach, as a better measure. Your saddle should always be the same distance behind the BB (the old plumb line from knee thing). This is achieved by a combination of set-back, and the angle of the seat tube. The trouble with ETT is that it is only effected by the STA, without taking into account set-back. So you can be misled.

So you buy an aggressive frame with a 74.5 STA, with your perfect ETT of 53cm. Place your saddle on it, pushing it back say 1.5cm more than usual because of the steep STA, and bang. Your reach is 1.5cms more than the ETT hinted at.

You dont need to do the trig. Just be aware when using ETT, and quickly check the seat tube angle.