There is no one ideal position. Every rider is different, and now there is data to show that even for one particular rider, the best position changes with yaw angle. So work from the general guidelines and test and modify from there.

Originally posted to RBR by Andy Coggan:

"Not to be disagreeable, but I disagree. If my goal were to set myself
up in an aero position with minimal drag and I didn't have access to a wind
tunnel, I'd just drop the elbow pads far enough down below the saddle
that my shoulders (acromion process) were within a couple of inches of
being level with my hips (head of greater trochanter), move the elbow
pads in to where my arms were as narrow or perhaps narrower than my
thighs when viewed from the front, tilt the aero bars up
ever-so-slightly, and keep my head down. I'd then go out and ride the
bike - hard - in that position and see how far foward (and thus up) I
needed to move the saddle to where my thigh-torso angle was similar to
the "working position" on my road bike. I'd then ride the TT bike for at
least one hour - hard! - each week for at least 6 weeks before any race.
Sounds crude, I know, but for a flat TT this neandrathal approach will
probably get you to within about 1 km/h of your maximal speed."