My first wheelbuild went poorly, or why experience helps

So I recently attempted to build my first wheel, and it went very poorly.

I go according to various documents and it starts taking shape: Except for one part, around the seam of the rim, i try for about 6h and I am getting nowhere.

Enough: I hand it to a pro.

First thing: The tension is all wrong, the tensiometer that is a clone of the park tool but made by a very large bicycle equipment store here in Germany is just plain wrong. We check my usage, the table, it just reads 120kgf and his professional calibrated device reads 60kgf. Ouch #1!

He looks at the rim, not the most expensive but with a good reputation around the internet, and says he has only seen such a poor seam on a new one once in the last 10 years he build wheels. Ouch #2!

This is the end of my rant, my first build went very wrong without me doing anything wrong. Without the help of someone experienced I would have never gotten closer to a solution