A P-bass having a larger pg (than a J), you can loose a bit of weight but not as much as you'd hope though. To optimize that I do a honeycomb pattern using a drill press with various sized Forster bits. I drill down to 1/4" and position the holes as close to each other as possible. The honeycomb keeps it stable so use every available space under the pg.
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Much conjecture about its effect on sound/tone - but the only time I noticed it was when I did a much more involved chambering. This is the late '80s alder J body I mentioned. I routed the top down 1/2" and added a 1/4" maple cap also chambered (to 1/8") in-line with the alder holes.
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...and more recently this one - this is the top which is flamed maple. The one slab mahogany back was chambered in-line with the top holes. Depth of the body is 1/1/4". When doing the whole body, one has to be aware of scallops and belly bevels so as not to drill to as deep in those area's.
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