Vary equivalent resistor linearly between two values
So you need a 215R POT. If you can buy a 250R pot it may be close enough with a 1.54K in parallel.







simulate this circuit &ndash; Schematic created using CircuitLab





NOTE: You can get pots closer to 215 than 250, to use with a larger resistor and get closer linearity, but they are uncommon and cost more.