simple question... when measuring the trigger pull on a gun, where do you place the end of the weight on the trigger? High, low or in the middle?

I don't know what the "experts" do, but I place it at the center of where my trigger finger rests. Then I pull back in the same plane my finger would pull it. In other words, I mimic my own trigger pull.

IMHO, I think that wherever it is placed should be consistent. That will provide accurate changes over time. Precisely where it is placed, within reason, is probably less important than consistency.

I also mimic where I place my finger and try to be consistent. My finger is almost always below the middle or about a third of the way from the bottom.