That was going to be my next question; How do you know when the battery is getting close to the max drain with a mech mod? Looks like it's when the vapor/taste isn't what it should be (or what you think that it should be)?
Back in the day, voltage drop off was blatantly obvious; used higher ohm coils. With a good 1.2 ohm coil build, drop off is less obvious because instinct causes you to press button longer to get good vape. If you pay attention you can get a feel for lower batt voltage. My 11-21-11 18350 batts still perform good; 3.58v after vaping 2ml aqua dry. My new 18350 are 3.66v after vaping aqua dry. Apparently draining batt just below 3.2 volts a few times doesn't do significant damage.