Since your shoulders can reach the beginning of the top third with a bit of a stretch, you should still put it within the top third.[A] If this is not so, it should be placed at shoulder height of a normal person.[B] Practically speaking, if the top third of the doorway begins above 5’ 11” (1.8 meters), the mezuzah should be placed at a little above 4’ 6” (1.4 meters), which is average shoulder height.
 Misgeres HaShulchan, comments to Lechem Hapanim, Kitzur Shulchan Aruch 11, cited by Mezuzot Beitecha 289:23.
 Kuntres HaMezuzah 289:20; Agur B’ohalecha 12:8.