This reminds me of your description of tourist clusters in the past, and I think your guide is a good rule of thumb.
That is; 5 blocks from the tourist center and your in the thick of it, 10 blocks away and there is a 50 percent drop off, and at 20 blocks from the center you get into the normal culture.
This is etched in my mind somehow. What I like is that 20 blocks is always an easy walk for me. So it's quickly avoidable.
Question for me is always to what extent the locals (60 percent as you mentioned) have deviated from normal as a result of significant tourist influx. And where is Khao san rd on that dial?
This is a very fuzzy cultural observance and difficult to see or know exist. A person has to walk in and out of the two areas enough times to see there is a real difference.
For sure a person has to be judgemental and take the PC glasses off.
Very good observation, linking the two rather nubulous post together.
I continually try to think of a way to explain the differene between the real country, and the suddenly changing country, or rapidly evolving culture of a tourist area.