Here is a description of the method I use for establishing a rider's fore/aft position on a TT or tri bike. I'm interested to know what you fine people think about it.
The objective is to ensure that the rider is able to apply all of their body weight to the pedals.
With the rider positioned on the aero bars and one pedal in the forward position, I then make sure that the pedal won't move when the rider pushes down on it by holding the rear brake lever myself or by holding the rear wheel. The rider must then lift their hands and elbows off the aero bars by pushing down on their foot. If the rider cannot do this, or moved their saddle position first, then their centre of gravity is too far forward and they will not be able to apply all of their body weight onto the pedals (because some of it is leaning on the bars regardless of how hard they push on the pedal). This is often influenced by the saddle fore/aft as well as the reach and drop to the bars.
Ideally, when the rider pushes down on the leading pedal their weight will lift slightly from the bars and saddle at the same moment.
Correcting this will often leads to a position that is less aerodynamic but more powerful. I always consider applied leverage to be more important than aerodynamics.