Problems with deck rail installation

I am installing a Timbertech Radiance Rail system. It is great and generally easy to work with. I really like the fact that all drill holes are the same size as are all the screw heads. I am not a carpenter, just a homeowner with basic skills.

Today, I had to put together a section of the rail that is on a 45 degree angle. Not wanting to mess up the product, I did a mock up with a 2x2. After a lot of trial and error, I finally got the length right. Her is my question -- when I installed the rail, the angle was not quite right and there is gap on the ouside part of the rail. the portion on the front is flush, but it looks like the angle was slightly off. I can't tell if it is because the posts are slightly twisted. Any thoughts? Caulk will cover it, so it is not a huge, deal, I am just trying to learn how to do it better as there are 3 more sctions with the angle.

Also, is there a good trick for measuring the length on the 45 angle?

Thanks.