I seem to make the “top deep” mistake myself.  I usually end up packing the base of the cavity to bring the pickups back up to a sensible height.

All I can suggest is measuring them.  Work out how much of the pickup will be proud of the top of the body, and then cut depth of cavity to suit.

You don’t need much depth.  As I should keep reminding myself!!

I’m glad you have asked this question because I have just bought a P90 and will be installing it into a project soon. I have never fitted one before and would probably have fallen into this trap also. I hope more people answer. 

My Yamaha RGX A2s ‘AIR’ guitars and my semi-hollow body Yamaha AES 1500 use twin coil humbuckers in a P90 sized pickup cover. They’re identical to the Dimarzio P90 range in form factor.

The RGXs have a 21mm rout, while the AES 1500 has 11mm down to the supporting braces cutaway.

I also have a vintage Gibson P90, which I suspect came out of a dog-ear cover originally, and a Dimarzio Super Distortion, both fitted into a Strat body which was routed for me by Manchester Guitar Tech. The Gibson P90 bobbin is quite shallow, but it fits OK in a standard cover. It sits low in the neck position with a 20mm rout. The Dimarzio Super Distortion is quite polite by modern standards but it’s another full depth humbucker in a P90 casing: high in bridge position with a 24mm rout.

All of which is to say, I reckon @TV101 has the best answer. Check twice, cut once.

Where have I heard that before?

The other thing worth keeping an eye on is the length of the mounting screws. If you’re using soap bar style P90s the mounting screws go through the middle of the bobbin and bolt straight into the wood below.

Check how long the supplied mounting screws are. It can be difficult to source extra long ones if you rout too deep and then want the pickups sitting high in the body.

Also, probably need a 1/4 inch wide router bit for the corners which I do not have. I simply just sand the corners a bit with a dremel to make it fit accordingly.

22mm for wilkinson p90s enough spring tension with minimal struggle in installing. Nipped the springs a touch.

@jamesbisset yup, it all factors in, from screws, pole height. Essentially, no nominal depth like TV said just gotta measure. Filling up a cavity can be a pain and the fact that you did not have to do it feels good. Thanks all, needed that.

Stewmac re-did their routing templates somewhat recently and in addition to being thicker they also include handy routing depths. Using those measurements as a guide seems like a great starting point, cheers!