The above has looked at a very simplified voyage estimation. This calculation just begins to touch the surface. Plod along and you will find that there are every more details you can enter to your voyage calculation.
There are some great tools our there that will help you automate your calculations. One that I like is called “NETPAS Estimator” found here: http://www.netpas.net/products/detail/ne
A free trial of the software exists. Here are a few screenshots: