It's the new way, apparently. Makes sense!

See more here - http://www.sicklines.com/2008/12/18/reach-and-stack-a-good-idea/

Centre of the BB to the top of the head tube - horizontal and vertical = reach and stack.

You need a plumb bob.

Here's what numbers come out, unsagged on a Ragley mmmbop, Ti or Blue Pig.

UPDATED ::(I got a new version of the very ace BIKECAD which gave more accurate dimensions).

16in - 23in top tube, 73deg seat angle.
reach = 390mm, stack = 576mm (140mm fork)
reach = 378mm, stack = 584mm (160mm fork)

18in - 23.5in top tube, 73.5deg seat angle.
reach = 408mm, stack = 576mm (140mm fork)
reach = 396mm, stack = 584mm (160mm fork)

20in - 24in top tube, 74deg seat angle
reach = 424mm, stack = 581mm (140mm fork)
reach = 413mm, stack = 589mm (160mm fork).