I also didn't notice the "Free Preview" on the progress bar until reading about it in this thread.

As distasteful as it may be, an actual heading or otherwise strongly formatted text note near the top should suffice for most people.

It looks like you cut the preview from 46% of this chapter to just 22% of the chapter... I went back to look for a quote from about 41% of the way through to explain to another commenter why the style is so effective and ran into the paywall earlier than I had my first time through. Just FYI, the content before 22% isn't strong enough to sell the preview. It has to get to the questions where you're asking for intuitive guesses on the real mathematical expressions to make it clear how valuable this is.

Honestly, you should just be giving away this whole chapter. If you did then I could forward it to colleagues, let them learn this one concept and suggest they get the rest of the info.

As it is I really can't do that. It'd be like giving a friend a flyer that was shoved under my door.