Thank you DaleSpam,
That helps a bit. 'The pressure is up to 1546 mm H2O and the volume is 300 L' so if I take the middle of the of the points of pressure as 1373mm,
-1200mm at the top edge and 1546mm at the bottom edge, middle = 1373mm,
and multiply that by 300 litres, which = 411,900...