On Saturday, 9 January 2016 22:51:05 UTC, wrote:
On Saturday, 9 January 2016 16:22:06 UTC, Roger Mills wrote:
The new fill valve is adjustable for height. It has a "critical level"
indicator and the blurb says that this must be at least 1" above the
overflow level.

I was right.

https://www.wras.co.uk/consumers/res...ritical_level/

The physical or piezometric level of the fluid in any part of the receptacle a minimum of two seconds after closing the water inlet, starting from maximum water level.

Interpretation given in Regulators' Backflow Specification - backflow prevention arrangements and devices: terms relating to backflow prevention

Owain