There are of course many factors you could include. If you do an excel sheet it is pretty trivial to include all sorts of numbers. The advantage of this approa

There are of course many factors you could include. If you do an excel sheet it is pretty trivial to include all sorts of numbers. The advantage of this approach is that instead of just guessing you'll be naking an informed decision based on an assessment of the underlying assumptions. In my opinion this is hugely superior to just guessing.