144000 Units of P0 = amount of P0 material converted in a Basic Processor to a P1 material. I.e. if you have 1440 m³ of raw material (P0) you have 24h of production in reserve.
960 Units of P1 = amount of a single P1 material converted in an advanced processor to a P2 material. I.e. if you have 364.8 m³ of each P1 material you have 24h of production in reserve.
I started off shipping everything to the production planet. I'm setup with 4 extraction planets and 1 manufacturing planet and I'm finding the planetary storage to be a bit of a limitation. I'm thinking of storing most of the P1 stuff off planet at the POS and only ship down what I plan to produce in 20h batches in order to streamline the procedures a bit. Once I get a hang of it I may go to 44 or 92h batches. 4h swing so things finish producing before I want to start the next batch so there's much less drift (i.e. couldn't start a new batch because the previous batch only finishes 1h after I go to bed). We'll see.