Although the descriptive specifically states from3 Countto the Nuke Plant, it is possible to unlock the achievement in either direction.
The Achievement starts when jumping off any part of the 3 Count or Burns Hill Reactors roof, including the top of the smokestacks or cooling tower. The Achievement is awarded when approaching the other rooftop, but it is not necessary to actually touch the roof.
The only feedback regarding this Achievement is the "Achievement Unlocked" Message upon completion. There is no in-game reward for completing the Epic Jump Quest.
There is no in-game feedback for starting, completing, or failing the attempt, so it is not possible to know whether the attempt is still active, or when subsequent attempts of the Epic Jump Quest are successful after the first completion.