We really should not be made to BEWARE. ...
I agree, but it's not a new issue, and so far there is nothing in the rules to allow the alternate volume measurement, or to slightly reduce the weight req to allow for it.
Anyhow, the volume should be about 2.3 liters for a 5lb extinguisher, and 4.6L for a 10lb'r.
Oddly enough, that is what is MARKED on the AFFF bottles (2.3/4.6), but for some reason, the weight does not line up with the volume. IIRC, the 5lb is 6oz light, and the 10lb is 10oz light.