Estimating basket volume

I plan on making a couple of sealed subs using a pair of the legendary Aurasound NS15 woofers. I’m decidling on the box volume but I need to make an allowance for the huge magnets and frames of these woofers. Is there an approved method for estimating the volume displacement within the box?
 
I don't think you would be too far out if you simply imagine the woofer as a solid cone as far as its volume displacement is concerned.

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The volume occupied by the woofer can then be calculated from the equation for the volume of a cone:

Woofer Volume = (1/3) * 3.14 * (Woofer Radius)^2 * (Woofer Height)

This calculator also allows you to treat the woofer as a truncated cone: https://www.omnicalculator.com/math/cone-volume

I've started the ball rolling at least. ;)