How do you determine the odd angles, when doing baseboards?

I am laying down Baseboards and am having a hard time making the angles match. What do you do?

The angle of the two cuts have to total to the angle of the turn. For example you could cut two 45's for a 90 degree corner. If the wall turns at a 45 degree angle, two 22 1/2 degree cuts.

The problem most people have is not cutting the material if they have a chop saw ot a table saw, it's the measurements. The distance on the wall is the inside measurement but you have to add the thickness of the material to allow for the overlap where they're joined. For example, a 90 inch measurement on the wall would be cut to 91 inches if the material is 1/2 inch thick and you're cutting 45's on the ends.

You have to be very accurate. A gap that's a tiny fraction of an inch will look really bad.