To be honest that's something I haven't fully put in to practice as yet but you can get all sorts of free apps to take any hard work out of the calculations......that aside if you focus 1/3rd of the way into the subject then you probably won't go far wrong (that seems to the general consensus!)
http://www.techradar.com/news/photography-video-capture/8-essential-landscape-photography-tutorials-1069997