In general if you can you want to keep the coilheads submerged, but that's ideal. If that's not an option just start giving it more time between hits, as in no chain vaping past that point. The juice will get wicked up to the top but wicking takes time. You should be perfectly safe to vape till you can't see anything when you hold the tank vertically. If you turn it upside down there will still be juice but don't go past that point as then you won't even have juice touching the bottom most hole and there is no wicking going on then.
Pushing the limit will lead to dry hits, dry hits lead to burnt cotton, which basically means you're killing your coil. So for best lifespan keep it submerged when in use. But, again just give it time once it gets lower and you'll be fine.
