I'm not 100% sure I'm right.

We use a Psychrometric Chart to solve this problem.

A Dry Bulb of 90F and a Dew Point of 70F would give us a Wet Bulb of 75.6F, a Relative Humidity of 52% and Enthalpy of 39.55 BTU/lb.

A Dry Bulb of 65F and a Dew Point of 58F would give us a Wet Bulb of 60.6F, a Relative Humidity of 78.1% and Enthalpy of 26.8 BTU/lb.

39.55 BTU/lb - 26.8 BTU/lb = 12.75 BTU/lb.

I attached a copy of the Psychrometric Chart I used. Hopefully it makes sense, it's been 20 years since I used one