The Base Power of Return is calculated like this: Happiness/2.5.
A base Happiness of 150 would be 60.
A base Happiness of 199 would be 79.6(rounds down to 79)
EDIT: The power would therefore be between 60 and 79 if the Happiness was between 150 and 199.
Max power of Return is 102 at 255 Happiness.