Hi Dave,
Thanks for the post, and welcome to the forum!
I would say no less than 8V on the SLA.
Simplest way to do it is attach an 8.2V zener cathode (half watt or even 100mW is fine) on the positive supply, wire a 470R resistor to the anode end, then attach the other end of this resistor to the anode of a green LED (3mm or 5mm to taste, anode is longest lead). Then connect cathode, shortest lead of LED, to ground.
Use this as your indicator LED when you switch on your Paris headamp.
If the voltage of the SLA falls to 10V from the nominal 12V, the intensity of the LED will about halve. At 8.2V, the LED will actually go out, even if switched on, indicating dangerously low charge.
This will be a useful visual indicator, and will give you a tracking method viewed from a distance.
Hope this helps, happy listening!!
BTW, a German hobbyist measured the Paris headamp and found its noise levels at -143dBA. This is astonishingly low, the lowest he measured in a number of headamps he tested!
Cheers,
Hugh