Both ChickenHouse and Dera are right.
The correct way to adjust the mixture for takeoff at high DAs is to lean until the engine just runs smoothly with T/O power set. This will inevitably be slightly rich. At high power settings however, slightly rich is less potentially damaging to the engine than slightly weak.
I would suggest that the figures shown on the placard are a guide to approximately what you should expect, rather than a figure that you should set, but I agree that it would be better if it showed density altitude rather than airfield elevation.
MJ
