I've got this pinned on my lighting board, don't remember where it came from.

But recently I read somewhere that it was supposed to be 36".
I'll be interested to see what the experts say.

As low as possible without obstructing line of site for your family & guests, so the best height depends on the height of your family & guests. I'm 6'2", so the optimal height for pendents in my house is ~6'1" from the floor.