from a thread regarding frame geometry. the chat
drifted towards the issue of linear measurements:

the head angle - choosing it as a "control" - doesn't
tell you anything. considering front center distance
allows you to determine the rider's placement between
the two wheels. i'd rather hit the fc measurement and
get the rider's upper body in an optimium position.
altering the head angle by minutes can mean the difference
between one size stem or another; a few minutes either
way can afffect alot of things.
i don't think it's easy to grasp this unless you study
alot of frames and/or are riding alot yourself. it's simple
to grasp the "73 degrees equals ???? road bike steering
blah blah blah" because there is a multigenerational layer
of magazine road tests and manufacturer's catalogs that only
list an angle and not all thre other linear distances.
an angle is a means to an end, it's not the end.
e-RICHIE