How do you miter moulding where the horizontal chair rail meets a corner that turns up at 45 degrees staircase?

You dont.External corners are mitred,while internal corners are scribed.

A 45 degree turn is an angle

You need to make 4 cuts @ 22.5 You will have 3 peaces of base to make the corner. the center one will be small about 3/4". Be careful. The other way to trim round corners is to get some out side...

That would be a Complementary Angle.

Wheelbase length.