How to Protect an ADC Input from Overvoltage
If you simply add a fixed resistor between the top of the trimpot and the 15V source, you gain two advantages. First, you can limit the maximum voltage applied to the ADC as you desire, and second, you get better resolution in terms of calibrating the exact divider ratio.

For example, if you have a 2K trimpot and a 13K resistor, the maximum voltage that you can get at the wiper of the trimpot is 2V, and now half the range of the trimpot covers a 1-volt span, rather than just 1/15 of the range.