Need2Know,
I believe your number is off. Your liter to gallons calculation is correct, but 360 POUNDS!?!
If you have a local hydro store, call them with the number of pots and size you have, the grow medium you want to purchase and have them recommend how many bags.
I just cleaned up 12 x 7 gallon pots of dry, 50/50 coco/perlite for my winter grow. I doubt if all 12 pots, 84 gallons, combined weighed 50 pounds. Somethings off.