I just tried spiders to see what I am doing. When I have a gap between the fleshy pad of my thumb and the neck, my thumb is pretty much at a right angle to the neck. When I brought that part of the thumb closer to the neck, so less gap, my thumb pointed more towards the headstock.

Both worked though and I had no discomfort either way. There might be a right way, I just go with what gets me playing the tune and don't think much about it.