Knee position while on the aero bar
Proper fit in an aerobar posture is designed to allow you to race well. In order to race well you will need to be comfortable enough to produce power, to reduce aerodynamic drag, and to handle the bike well and safely for the duration of your race. If your fit otherwise meets all your needs, you shouldn't be concerned about where your knee is positioned relative to the pedal. Under UCI regulations (see section 2 of chapter 3 here), the nose of the saddle should be a minimum of 5 cm behind a vertical line through the bottom bracket; morphological exemptions can be made to bring the nose of the saddle up to the bottom bracket. Some riders do not compete under UCI rules and the saddle can be forward of that; and many time triallists will sit only on the front half of their saddles, effectively bringing them even farther forward. The forwardmost position of the pedal will be determined by the bottom bracket and the crank length. Some riders use "short" cranks to improve their aerodynamic position, and that will also change the position of the knee relative to the pedal. The location of the cleat and the length of your foot will also determine where your knee is positioned relative to your toes. 

Optimize your fit to achieve your other goals. Don't focus solely on where your knee is positioned.