I would do a 500k volume, one 500k no-load tone (for the bridge), and one 250k no-load tone (for the neck & middle) - keep in mind, I do like a brighter sound so you might not want to do no-load tone pots.
As Stephan mentioned you can do a resistor trick to get the same tonal shift as using a 250k volume - add an autosplit for the bridge + middle setup, the resistor for anything that isn't the straight bridge hum, and you're golden. You'll definitely want a superswitch for this setup though, and while they're kind of a bitch to wire up you can do some really cool stuff (f.e. the Areas are a bit lower-output in hum-cancelling mode vs. when you split them), so to even out the pos. 2/4 volume drop you can set it to split the Areas there.