P90s are awkward to get sitting nice and parallel to the strings - normally because the bottom of the routs are parallel with the back of the body and the neck is angled.
I bought a selection of adhesive backed neoprene foam sheets in different thicknesses from eBay for this purpose on my P90 equipped guitars. They've come in useful for mounting other types of pickups too - P and J bass pickups for example. I think mine were A4 sized, but you can get smaller sizes. They typically come in 2mm stepped thicknesses, so a strip of one at the front and another 2mm thicker at the back of the pickup normally levels it all out nicely.
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