If you are seeking a quality job when routing out material to exacting measurements you need a template guide.  I bought this for a very expensive deck job.  The stair stringers were rotting and had to be replaced.  I made a template and replicated the existing stringers. Everything fit great.  You'll have a bit of time making your template but it's fast and a no-brainer after that.  When you are repeating the same cut over and over again this will make you shine.