@wingfield - to quote the EPA - “your mileage may vary”. It depends on how thick the walls are and are they on a very small angle (which makes the signal attenuation more) to the straight line between the two points.
I believe the 300’ measurement is above ground (another big signal strength eater) and clear line of sight (i.e. no brick or Hardy ™ walls).
I’d buy the flow meter from where you can return it if the signal strength isn’t there. One can put the batteries in the flow meter and test it for connectivity before doing the actual install. The flow meter uses 900 MHz frequency so it will go farther than the 2.4 GHz WiFi frequency.