I find myself swapping out at around 3.6 - 3.7 volts too, on my tube mechs, regardless of whether I'm using a single coil RDA at 0.5 ohms, or a dual coil RDA at 0.25 ohms.
On my squonkers the RDAs are more restricted, but still 0.5 ohms (I use the same 24g Brochrome build in nearly everything), I can run a bit lower to 3.4, and my MTL Squonker (Berserker MTL RDA on a Pico Squeeze) where I use Vandy Vape Ni80 MTL superfine fused clapton wire at 0.8 ohms I can often find myself running down towards battery cutoff at 3.3 or 3.2 volts.
All my squonkers (Pico Squeezes, Wismec Luxotics and an Asmodus Luna) are unregulated but have low battery protection.