Nice piece of wood, and nice work from it.
Remember every cut is practice.
Thin is as thin as you want it to be. Years ago I got hung up on thin and ruined a bunch of nice pieces going too thin, and ruined some I'd finished by deciding they needed to be thinned down. Too thin as Dave said is when the inside and outside meet.
I've seen lampshades that were turned thin enough to see through and pieces that were indented as opposed to hollowed. Who is to say what is right? The turner. Go with what you like and are comfortable with.
FWIW, I found looking at pottery helped my eye a lot with lathe work. Potters have the ability to redo and change and start over that we'll never have. Looking at lines of their bowls helped me see the lines that I like for different sizes and height ratios.
I hope to have some of Craig's tools. Everything I read about them is almost the same - quality and efficient. Did you get them handled or choose to make your own?