most clearos do best between 7.5w and 8.5w. The BVC (nautilus mini) heads are a bit different than most standard heads. Some have vaped them up to 20w with no problem.
the general rule is do what works for you: start at a lower wattage/voltage. then gradually increase it, taking a few puffs to try it at each stop. if you start to get dry/burnt hits, its too high for your draw. Some times heads are just crappy from the manufacture, so its possible you just had a bad head.
hth, good luck, and most importantly: dont get discouraged!