The theory says 250K for single-coils and 500K for humbuckers (hotrail included).
But the experience and rule of thumb can say also anything different!
It depends a lot on if you like darker or trebly tone, and if your guitar is darker or trebly (the fretboard wood maybe has the biggest influence).
The 500K (or higher) pot will give a more trebly fresh tone, while a lower pot (such 250K, but you could use also something in between) will give a slightly darker tone.
Do not expect a very big difference anyway. The hotrail pickup is anyway rather dark, so maybe 500k is ok. Enjoy and experiment anyway! Do not forget that you could also add a resistor to the pot to change the value...