Redmine is seemingly complex. It is. However, when it comes to Nginx, it isn't.
This is very nearly a drop in configuration. The only thing you should need to change will be the root location, upstream servers, and the server name.
In the above configuration I used upstream connections to ruby processes. They were bound to ports 8000-8002. Explaining how to set this up is explained on the redmine website and is probably best left for there. If there's enough request I'll try to compile something.