0' mean in-phase, 180' mean out-phase.

please view the pictures as below :


let's say this is in-phase sound wave


let's say this is out-phase sound wave



when in-phase sound wave mix with out-phase sound wave, it will be cancellations with others (look at the red line sound wave)


trust ur ear, it can give you the correct feeling, when the sound wave don't have cancelations, you will feel more bass into ur car, mid bass and sub bass with come together in once times, and the sub bass will come from front, not rear.
If the sound wave cancellations, u will feel the bass is less pressure / dB, and the mid bass never combine with sub bass together.
Don't affraid to try adjust it few more times, 0' or 180'。。。。。。just try it and listen it

no any formula teach u how to tune, just trust ur ear, dun affraid to try more and more times