Calculating resistance value for voltage range
It takes a certain minimum voltage to make a Nixie tube light up.
Once it is lit, the current controls the brightness.  More current = brighter light.
More current also means shorter life span for the tube.
You want a balance between useful brightness and long life time.
The ignition voltage will push a rather large current through the tube, making it rather bright and shortening the lifetime of the tube.
If you know the rated operating current of the tube, then you can figure out a series resistor.
You need the source voltage (Vs,) the operating voltage (Vo,) and the operating current (Io.)
The series resistor is then given by \$ R = \frac{V_s - V_o}{I_o}\$.
The required information should be in the datasheet for the tube.
If you have the datasheet, though, then you probably won't have to calculate anything.  The needed resistor value will be given.
Like in this example:

The &quot;Test conditions&quot; give the operating voltage and the value of a series resistor.  Problem solved.
It also includes the typical operating current if you'd like to do your own calculations.