The Achievement starts when jumping off any part of the 3 Count roof, including the top of the smokestacks. The Achievement is awarded when approaching the Burns Hill Reactors, but it is not necessary to actually touch the roof.
There is no feedback given while attempting to complete this achievement, so it is not possible to know whether the attempt is still active.
The only feedback regarding this Achievement is the "Achievement Unlocked" Message upon completion. There is no in-game confirmation or reward for completing the Achievement, so subsequent successful completion of the Jump Quest does not give any feedback.