I have to pass this on to you all.
Having had no data to the actual discharge allowable on a given 3S battery I have had more help from a more knowledgeable friend.
He has said :-
A full cell is 4.2v/cell and a discharged cell voltage under nominal load is 3.6 volts these still remain as the established and de-facto values to use. So for a 3S battery/system: -
1st level protection voltage (under load, not a recovered voltage) would be 3 x 3.60 = 10.80v.
2nd level protection voltage (under load, again not the voltage from recovery) would be a nominal 0.1volts less, so 3 x 3.5 = 10.5 volts, more usually 10.6v. to give a slightly increased margin.
This gives you a system that will not destroy your batteries and when you get into trouble, have sufficient power reserve to undertake an RTH or safe landing from altitude, noting how long it takes to undertake a controlled descent, which of course consumes power in doing so.
Reminder that battery voltage is a very rough estimate / measure of capacity. You could pick-up a used recovered battery and think it is at say 50% charge using the voltage method and find (the hard way=crash) that it has perhaps 5% charge left, many people have done this, myself included, this is why I now segregate used batteries from charged batteries so then can never be mixed and used again.
So, I have to advise you all that to consider battery longevity a base line of 85.5% (10.80 v.) for the first level warning, and approx. 83.52% (10.6v.) for second level warning.
This will, of course, give a slightly shorter duration than my final flight trial but will save the battery.
However, I am going to keep the settings as per my final trial flight and see just how long the batteries last - just out of curiosity. I may be sometime ...........
Pete