No-load voltage alone can be an indicator of remaining per cent of full charge, but with a considerable margin of error depending on such variables as battery age, condition, number of charge cycles already experienced, temperature, etc. In general, from what I've read in various places, a no-load voltage of 12 volts on a healthy AGM house battery indicates that it's near the 50 per cent discharged point, and lots of AGM makers and sellers suggest not going below that point on a regular basis. If I find we've dipped below 12 volts and we're parked for the night, I either turn everything off except the fridge, or run the generator for 20 minutes or so. As near as I can tell from my reading, discharging an AGM battery to below the 50 per cent level now and then may be okay, but doing it repeatedly will shorten the number of charge cycles the battery can handle - i.e., in my simple layman's terms, my battery will die sooner.
1993 E350 SMB, 15M, 84k miles - a cheap date
2006 Toyota Prius because we like clean air
2002 Honda Odyssey - can haul lots of stuff
1972 Mercedes 350SL with 4.5l V-8, just because
Wag more, Bark less, Play well with others.