question

Unfortunately, voltage to state of charge relationships only apply when AGM batteries have been completely dormant for several hours, i.e., just sitting there - no charge or discharge.

It's difficult to specify an arbitrary voltage because voltage drops under load.

Your 280Ah (840Ah) batteries are likely rated at a 20 hour discharge, i.e., 840Ah/20h = 42A. Anything above this means you have less than 840Ah.

Assuming you're pulling a 800W load, that's a 800W/12V = 67A load. The moment you unload it, the voltage will rebound to a higher level more indicative of SoC than when loaded. If your battery is at ~60% SoC, and you apply a max load do it, do you want it to shut off because you've specified too high a votlage cut off? Probably not.

The best solution is to get a BMV or smart shunt and track the true SoC. Get it down to 50% and hit it with a max load. After a few minutes, note the battery voltage and then set your cut-off to something a little below that.