Hi Gus Chair rail inside corner miters should be about 45 degrees. However, these inside corner joints should be coped...not mitered. The two problems with inside miter joints is that they tend to pull apart (spread) when the wood is firmly nailed in place and second, as you point out, the angle is usually not a perfect 45 degrees anyway, meaning you have to do some open joint filling with what my dad used to call "goofer-proofer".
I'd suggest that you go to the library and get a good book on finish carpentry, which should show you the technique for coping a joint. Its not too hard, and once you do a couple, it will seem easy.