This half full/half empty nonsense has always amused me. Some really daft psychobabble is spun around it!
I would say "The quantity indicator displays 50% contents".
Surely the actual answer should describe the progress achieved towards the final result? So, if filling something up then at 50% quantity it's half full; whereas if emptying it then it's half empty at 50% quantity?
Psychobabbling trick cyclists come just below estate agents and snake oil salesmen in my league of charlatans!