If this guitar is set up like most with P-90 soapbars (as opposed to the dogear P-90 type), there should be two pickup height adjustment screws that go through the pickup cover and the pickup itself. They should be screwed right into the body beneath the pickup. Look for the two screws that look different from the others.
In my experiences with P-90 pickups I've found that the neck pickup sounds best when cranked down to where the pickup cover is flush or almost flush with the pickguard/top of body. I've found the opposite to be true with the bridge pickup. It seems to sound best close to the strings. Of course, pickups will vary depending on manufacturer, and these adjustments can vary as a result.
Try a wide range of pickup heights and decide which sounds best to your ear.
I also like 300k CTS pots with P-90's, so long as they read 300k or better on my VOM. I've always thought that 250k pots sound a little too dark, and 500k pots a little bright (but not always). Another good compromise is to take a 500k pot and solder a 1 meg resistor from outside pole to outside pole on the pot. This reduces the resistance to 333k.