When laying out or cutting the stair stringers, don't think of angles. Your steel square, set to the rise and run, gives you the correct angle automatically. This is covered in How to Cut a Stair Stringer, section The Layout.
For the angle on top of the handrail post, lay a 2x4 down the steps, resting on the nosing of each stair tread and touching the handrail post. Scribe the vertical angle of the post onto the 2x4. Then set your miter saw or circular saw to that angle or just cut the 2x4 along the line you just scribed and use it to set the angle of the saw. Cut each handrail post with this same angle. For the ends of the handrail itself, use this same angle. Just be sure when cutting the ends of the handrail that you cut on the same side of the board so the ends are parallel to each other. In other words, don't flip the handrail board over between the end cuts.
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