You have 2 separate issues in my opinion
The first is that the angle setting may be a 1/2 degree off. The second is curved/bowed boards make it very difficult to get a curved line straight.
You are trying to do something using a circular saw which would be easier to do on a sliding miter saw or a table saw using a sled.
Now you have to "fix" what you have.
You can rip the boards into 5" pieces and glue them back together in a flatter configuration OR as suggested, "hand fit each joint to mate up closer. Miters are among the most difficult joints to make whether by hand or using power tools in my experience. The wood must be perfectly flat and the angle must be 45 degrees exactly. Then there is the issue of gluing them together without slipping and mismatching. Splines are sometimes used to help with alignment. This is not an easy thing to do even with good material and the proper tools....
The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)
Last edited by woodnthings; 05-27-2013 at 07:07 PM.