I don't know a good way to measure what Gary has described.(It is the best looking technique, by the way.)
The way I do it, is to take a bevel guage and lock it at 135 degrees.(90 plus 45) The easiest way to get that angle is to simply adjust the blade in your chopsaw to 45 and then put the sheath part of the bevel guage against your backstop(on the obtuse side) and then swing the bevel blade around until it meets the saw blade. Then tighten the wing nut. Voila! 135
Stand your piece up in place and then place the bevel guage on the other side of the corner. The blade will cross the base at exactly where it needs to be cut. You can't use the tangent because it will wind up short. Simply do this with each piece and it will be perfect. If the radius is relatively constant on every corner, you can cut the little filler piece and use it to check the corners before you stick them up. I just keep the filler piece in my pouch and use it as a failsafe. It usually works great.
Good luck.
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