Fill pressure effects how much gas you can cram into any given volume/cylinder size... when a 300 bar bottle is showing 150bar on the regulator it's "effectively" half full
Pretty much, yeah. Forgot to mention that you won't get the full deposit back if/when you don't want the cylinder anymore, can't remember how it works with hobbyweld but IIRC they just deduct an "admin fee"? It'll be explained on their site somewhere