Sightline question

I’m working on figuring out the angles for a little piano bench I’m making. I’ve decided on a rake of 4° and splay of 6°. Using Gideon Golden’s chair maker’s calculator I get a sightline of 33.6°. But I’m not sure from which line to measure that angle. In my drawing would it be line AB or line CB?

I’m working on figuring out the angles for a little piano bench I’m making. I’ve decided on a rake of 4° and splay of 6°. Using Gideon Golden’s chair maker’s calculator I get a sightline of 33.6°. But I’m not sure from which line to measure that angle. In my drawing would it be line AB or line CB?