[syhprum (OP)]

I have seen various figures quoted between 2.3 v and  2.4 v per cell assuming a near zero output resistance of the charger up until its current limited cut off.
I had in mind 2.33 V but would like to know the approved figure   

[Bored chemist]

It depends...
https://en.wikipedia.org/wiki/Float_voltage

[evan_au]

Just to draw your attention to one short section in the above link: "Temperature compensation".

Even after you know the chemistry of your battery, the ideal float voltage varies slightly with the battery temperature.

[syhprum (OP)]

I was surprised to see that a lead acid battery was used in a military aircraft I did not think they were expensive enough !