AFAIK the "phase" control is really just a delay, with a theoretical 180 degrees being the equivalent of 1/2 the travel time of the upper frequency. So for a subwoofer with a 100 Hz max 180 degrees would be 1/200 = 0.005 milliseconds (if I did the math right). 90 degrees would be half that or 0.0025 milliseconds.
If you are using a HT processor this should be dealt with by the speaker delay calibration.
If using this with high passed speakers the correct setting is the one when the inverted high passed speakers + sub have the deepest null at the crossover. Otherwise it should be set for the smoothest response below the low pass filter setting.