I'm not splittings hairs. It was me, not you, who said that it was probably a reasonable general rule-of-thumb.
I was merely reminding folks that it is a "rule-of-thumb" - i.e. something which should not be relied upon.
As I said, probably more important than the number of occupants and bedrooms is the rateable value of the property. I know plenty of properties of sufficiently high rateable value that appreciable savings would result from having a water meter even if there were more occupants than bedrooms - and your rule-of-thumb would 'miss' those cases.
As I stated, for any poor mug reading this thread , the rule of thumb is a guide. That’s all