I have just bought a couple of Delphi Freedom marine deep cycle batteries
for my service bank. Is there a recommended minimum voltage below which one
should not discharge? Also, if the nominal capacity is (say) 100Ah, is that
measured between standard voltages or is it measured from a nominal full
charge (with full charge voltage dependent on the charging system) until
totally discharged - ie 0v? Or is there a standard "never go below" voltage
which manufacturers use?

I have an intelligent battery charging system on my boat that charges up to
a threshold 14v with an asymptotically decreasing charging current, and a
battery controller with which I can set alarms to warn me of impending doom
on the discharge cycle. At what level should I set the alarm to get best use
out of my batteries?