ujm > Sefer HaChinuch perek 239
ujm, thanks a lot for the reference! This sefer – that I would probably never find by myself – helps me understand the issue. I continued reading this whole perek and found further excellent advice – that in cases where the listener is not getting it, not to do it, including a reference to Yevamot 65b – that you can fulfil either a mitzva of giving tochacha when needed, or not give one when it should not be.
I also notice that the next perek 240 talks about a prohibition of embarrassing. I wonder whether these two prokim are connected somehow.
Anyway, I hope this is a good illustration of a tochacha given the nice way!