Does this mean that I have to vape at least 15 W (with a maximum of 60 W) and 10 W (with a maximum of 26 W) to get an optimum vaping experience (and be safe) with that particular clearomizer/tank?
As these coil heads are a fixed resistance value, with fixed wire gauge and coil count in parallel (if any)... the only controllable elements you have are wattage, juice PG/VG blend, and within the limits of the atty and your lung power... air flow.
With most all Glassomizer type atomizers (using a production coil head), there is a practical limit to the amount of VG you can use before the wicking media capillary action diminishes... and you start getting dry hits. This variable can, to a certain degree depend on your personal vaping habits, the atty design efficiency...
and the coil radiant heat value.
Your wattage will determine that last value... coil radiant heat. Obviously, the higher the wattage, the higher that heat value, and the higher that value, the potential for more juice to be vaporized in 'X' time. There's a limit there too... in net coil surface area, which has and effect on total vapor volume and density, with a given wattage/coil temperature.
In other words, if your goal is vapor volume and density, use the highest VG content that the atty will function with continuously within your vape habit envelope. Vape wattage/temperature will likely be limited to your vape temperature tolerance, in concert with air flow.
If your goal is good flavor, then ignore most of the above and follow Mark's advice... start low and run it up until you get your best flavor.
