Gibson always used 500k volume pots with P90's, they sometimes used 250k tone pots on guitars like Juniors. So, 500k volume pot is the norm with p90's IMO.
I agree with all that, but longer scales have a thinner sound. I tried a 500k pot and wasn't impressed with the p90. I wonder if a 250k pot would have been better
Depends on the slot. In the neck, always 500K's. In the bridge I use 250K's, as P-90's are single coils, and are pretty bright with thinner mids than a HB.