Here's the Safe Vaping Power chart. Ohm's down the left side, track horizontal from the left to find the safe vaping zone, in light green, and find the voltage setting at the top. Google "safe vaping power chart" to find yourself a larger image
Find yourself some AW IMR batteries. Best and safest batteries for a low powered tube mod.
If you are going to use a tube mod where you only require up to about 15 watts or less then the AW IMR batteries will be perfect. If you are going to use a tube mod which can is adjusted with + or - watts then the volts will be calculated automatically by the device (This type of device is referred to as a "VW" device (Variable Watts).
If for example you want to use a Provari v2.5 then it can be adjusted with volts + or -. This type of device is referred to as a VV device (Variable Voltage). Voltages are always low, unlike watts. The provari v2.5's highest voltage setting is 6v. At this moment in time i'm only using 4.7 v on a 2.1 ohm coil.
The Safe Vaping Power chart is a good guide to use. If you are going to use good quality reliable batteries like the AW IMR's and are going to use a low powered tube mod then the Power Chart is a perfect reference. The worst thing that could happen if you set the power too high is your scorch the wicking material in your coil and over heat the juice (Dry Hit).