Well I am surprised that the UK habdbook leaves any room for doubt! My French version is quite clear;
The top of the sight glass is MAX and the bottom is MIN. that is clear;
(0.5 litres of oil, representing 10mm on the level, is the difference between warm and cold.)
The drill. Only check when the engine is warm - after a run. Leave for at least five minutes. Then anywhere on the sight glass is OK.