Android percent screen width in RecyclerView item

How would I get a RecyclerView's item height to be, say, 30% of the screen's height?

I can't simply use a ConstraintLayout or a PercentFrameLayout/PercentRelativeLayout, since those position child layouts relative to parent layouts, whose size might not match the screens.

Preferably, I'd like to do this in pure XML (and not have to use any Runnables to get the screen height dynamically).

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