Fall time measure at Labview

Hello,
I triggered my scope (Tektronix TDS2024B) using Labview.
I got with that trigger a capicator discharge formwave.
Now I want to measure the time when the discharge started until a spesific voltage that I can determine.
I know how to do it with my scope using the cursors (manualy),  but I don't know how can I do it when I trigger the scope with Labview and then measure it automaticly.
Best Regards,
Ariel.

Thank you for your answer...
I'll try to express my self better.
You can open my VI and see that my wave is start from something like 18V and then (after one second) it's start to fall until it gets to 0V.
I want to measure the the of the start of the fall (someting like 1 second), until 4.5V (someting like 1.15 second).
I just need it exactly and not an assumption.
I hope I expressed my self better this time.
Ariel.

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