Let’s start with a forehand grip. From here you get a full panhandle by turning the racket about 90 degrees. A partial panhandle is similar, but you only turn the racket about 45 degrees.
This is not an exact number, because you will need to use more or less panhandle, depending on the situation. But 45 degrees is a useful starting point.
In other words, the partial panhandle grip is halfway between forehand and panhandle.
As always, the grip should be relaxed and cradled lightly in the fingers. It should not be held tightly in the palm.