I can't remember where or when I heard it. But I've always used one and one half times the material thickness for the depth of a hole for pipe or tubing.

So a one and a half schedule forty pipe would be approximately three inches. (1 7/8" X 1.5 would be 2 3/4")

I don't believe I pulled this out of the air. I'm comfortable with saying it's something an engineer or someone close to one told me.

Am I wrong?