Do not use a convertible note. Widespread as it's use is, what is critical is alignment of incentives between the parties.

Both should be clear with themselves and the other, what their target return is; for angels a multiple of the amount invested; and for founders the increase in net worth they seek as a result of the businesses success.

Fair has nothing to do with it. The word is a smoke screen for lack of hard thinking about whether or not there is a basis for a deal between the parties.