Metric Accuracy

I am wondering how accurate is this metric?

My initial tests on a [very simplistic prototype](https://github.com/sergeychernyshev/ttvc-prototype) were OK, but now I am trying to test real website using WebPageTest and am seeing significant discrepancies between `TTVC` measured using this library and Visually Complete metric recorded by WebPageTest (using screen captures).

The `TTVC` metric is usually larger ranging from ~30% to ~300% higher than the time stamp recorded using screenshots in a few tests that I manually ran.

Is there an existing methodology that I can use to assess the accuracy of this technique?

Additionally, are there any known aspects of the page implementation that are particularly prone to throwing this metric off?