On the other hand, if the complainers are already going to complain
whether it’s three free chapters or ten… then might as well give the
fence-sitters more to go on.
True but people WILL complain if you give them 80% of the book and then charge them the full book price for the last couple of chapters. Sure the book might have been 100,000 words in total and it’s still going to be that long whether the cut off is 20% of the way or 80% however the perceived value of the latter is less since many people will feel cheated for having to pay for what they see as a small amount rather than viewing the product as a whole purchase.
So the placement of the demo point really is important. Too little and people wont buy it due to lack of interest or suspicion that the short length is hiding a poor story. Too long and complaints will arise about the above or just because they’ve become immersed without planing to actually buy it only to be cut off.