As stated above, flat is far more important that level. Hollow core doors are never flat. They have almost no structure inside them and the main surface will warp in a short time. A solid core door is a good place to start. Also the pool table top is awesome. I use a 4x8 foot granite slab, but it was costly. If I had not moved from California to Illinois during the time when homes were worth millions in Caalifornia I wouldn't have it.

The solid core door, followed by a piece of 1/4" thick glass will yield a surface that will deliver many years of quality builds. Place the ceiling tile on top of the glass and door and you are good.