Nothing like wasting a cut, averything seems right till you pull it out of the miter box and hte ange is reversed!
I just finished a kitchen, fairly easy, (it would seem), 5 inside corners 1 outside corner. I actually finished and didn't have to go buy more. I did have to cutr a slice and insert it however, wood putty and paint fix many ills. Hate to think if I were using cherry.
The best note I saw in here was to put it in the niter biox in the orientation it will go up in. then the 45 is obvious. It is when I try setting a reverse that I screw myself up.
As far as how to calculate angles, you first need to know the angle to be covered, in a standard 90 deg corner, just divide by 2, and you need 45 deg. With the compound woever you have to get the molding in the proper orientation, you can't just hold it against the box. __|__ As in this primitive diagram, you see the base is against the fence, the top is out. Holding this is tricky, but a good saw makes quick work.
For non 90 deg corners, for instance, the octgon room mentioned in an earlier post, if all it is a true octagon, then divide 360 deg by 8, getting 45 deg, divide by 2 giving 22.5 deg. This is your cut. Just remember to have the piece oriented properly. Have a helper nearby to help steady the piece.