Mart what do you mean?!!!!
"I read somewhere" does not equal correct.
"Just a rule" does not make sense, to me anyway.
Rules are what works (or should be) and the word rule needs using carefully because when applied to something which works, it becomes enshrined as "fact", yet hides all the other background circumstances. I prefer to work with flexible guidelines…
For example I said 10cm has worked well as a depth of compost for my salad boxes. That is because they contain rich compost and manure, are cropping slowly over winter, in a greenhouse. The last three things are all variables which could change the headline figure.