What is a Fair Rake?

[DeadMoneyWalking]

We've had lot's of discussion on what rake is beatable, but has anyone guessed at what a fair rake is for a cardroom?
Does it vary by location?

By fair I mean enough for the room to pay its employees a living wage and turn a profit. And low enough that good players can still win at poker.

For simplicity, assume the room has only small and medium stakes nlhe and lhe games.