How to pick resistor values for voltage range with potentiometer
As your post is not very clear, I will assume what you want to do is to control the output voltage of your IC with a Potentiometer. So, I would suggest you to use a circuit like this one:







simulate this circuit &ndash; Schematic created using CircuitLab

To calculate the values for the resistor, just replace R1 in your formula with R11 + Pot. Consider the voltage to be maximum (30V) when Pot value is 50k, whereas the voltage is minimum (12V) when Pot is 0. This will give you the following equations to solve:


0.6 x (1 + R11 / R2) = 12V
0.6 x (1 + (R11 + Pot) / R2) = 30V


However, the values you will obtain are not standard. Here are some ideas to get more practical values, so the actual resistors will be easier to find:


Use several resistors in series or in parallel to do the job. Ex: for R2, 3 5k resistors in parallel will give the value
Change the potentiometer: a 150k potentiometer will give you 5k for R2 (easy to find) and 95k for R11 (more difficult, but can probably be done using 2 resistors in series)
Replace R2 by a resistor (R21) and the Potentiometer. I didn't do the calculation, so I don't know if it would be easier or not.
Put the resistors in parallel instead of in series, as suggested by Tom in the comments