So being an engineer I will give you all a lesson to help you understand what this dew point thing is and how to determine what dew point to dry your air to.
So your dew point is dependent upon your air temperature. Now if you have an unheated garage in the winter and your compressed air may get to be -20F then you will need to have a dew point that is just lower than -20F. If you have a heated shop and you will never fall below 50F then you will need to have slightly less than 50F and you will be fine.
The dew point is the temperature at which the water in the air will condense out and start sticking to surfaces and then we have problems like misfires and consumables burning up. So it all depends upon the lowest temperature in your shop.