Fore/aft position depends on age, weight, condition of the rider, the bike, and the type of event.

TT bikes have the rider's CoG quite forward for aerodynamic purposes but the rider typically loses aerobic power.

If a normal road bike position has the rider having difficulty holding a stable, comfortable position, it is a good bet that their breathing capacity is reduced by excessive recruitment of muscle to stabilize the torso.

Bring a 5mm and mess around on a training ride until you are comfortable. I cannot hold the positions of my youth and do not even try.