Pretty sure what people said in this thread is right: 1 valour increase for 2 command stars of your general. Try it on the strategic map: stack a unit with a good general and look at the unit's valour in the top right hand corner of the unit picture. Take the unit out of the stack and examine the unit again. It will have a lower valour.

What I did not appreciate is El-Slapper's point, that the morale bonus from the extra valour due to leadership depends partly on proximity. I knew there was a morale bonus from proximity to the general, but assumed it was on top of that from valour (natural and leader augmented). I may check this out on the battle field (there is function key that shows your units stats, IIRC).