How thick does plexiglass/acrylic need to be?

I'm building my game table, and I'm getting ready to buy a 4'x6' sheet of plexiglass to use as a dry erase surface over maps, mats, etc.

But how thick would such a piece of plexiglass have to be in order to be easily removable for map/mat placement, but still strong enough that it's not going to crack any time I pick it up?

Any idea?

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