Concave Protractor?

Hi, this is my first post here so I don’t know if this is in the correct category or not. But anyway, my question is: how do I have the protractor tool measure an inwards angle with a cylinder (I’m modeling a rocket engine I will soon be machining). Thanks!

Where you posted is fine, but it’s not clear what you want to do.
Perhaps an image or better still a model would help us understand how to help you.

I actually think I figured it out. Thanks, though!