My wife and I are considering building a fire pit in the center of our 3,200+ sf PT deck.

I would lay some type of heat resistant, multi-layer base, then ceramic tile surface and then whatever the fire pit.

We are thinking perhaps a 3 to 4 foot circle shape, but are open to other ideas.

My problem is trying to determine the center-point of the isosceles trapazoid - wider at the front and narrower at the rear - 90 degree on the base.