Anybody willing to chime in on how far back I need to dig this mountain spring before adding the collection dam?

Located about 2800ft in far western NC. I dug back from the outlet and continued to find it essentially running through a tube clay cave. From my readings on the subject I was looking for a point where it emerged from some kind of permeable but solid medium like a border between strata with, which I figured would just be where the clay met the rocky soil, but it’s going into the hill and digging is going to get hard. I’m considering just cleaning up this hole and placing the collection here near the outlet visible in this video. Any opinions are welcome, as everything I’ve read had been kinda vague on how far back to dig.