This chart (right click and save to your computer to read it) gives suggested voltages for different resistance coils heads. So for a 2.5 that you are vaping right now, the suggested voltage is anywhere from 3.2 to 4.5 volts. As a rule, start around the lower end of the suggested volts and increase from there.
The idea of having a VV is that you can adjust the vape to suit you. So give it a little twist and see if you like it - you won't do any harm to either the device or yourself. In general, turning up the voltage will give you a little more flavor and vapor. If it starts to taste burnt or too strong for you, go ahead and turn it back down again.
It's all about the flavor and what works for you
So now you have the chart, you can see that you can purchase any resistance coil head and then adjust your voltage up or down to compensate to give you the vape you prefer.