I read the OP and thought that surely someone would have corrected him by now, but alas.
I'll educate you guys in DoTs.
So your worry is that you may apply the dot, and it ticks, making the final tick happen 1 second after your DoT falls off.
But get this, if you apply the DoT correctly, as soon as the next one falls off, you lose no ticks.
So it boils down to managing DoTs correctly. In summary, all DoTs (duration/3) ticks.