When afixxing the mezzus on the top third of the door post do you have to put it on the bottom part of that third? If not, how much leway do you have in placing the mezzauh?

Shalom, Thank you for your interesting question. In fact, the question you ask is a point of debate among the great early Rabbis. The Rosh (13th century) believes the mezuzah should be placed a handbreadth below the upper lintel of the doorframe, while the Rambam holds it should be at the start of the upper third. The Shulchan Aruch (Yoreh Deah 289,2) rules like the Rambam, and so the ideal place is at the start of the upper third of the doorframe. If the door is very high, one should place the mezuzah at shoulder height, even if that is below the upper third. The upper limit is that it should be at least a handbreadth (8-10cm) from the top of the doorframe. Within this limit - between the beginning of the top third up to a handbreadth from the top - it is perfectly kosher. May you merit to affix mezuzahs to a faithful house in Israel, and may the blessings of the mitzvah bring holiness to all those who pass through its doors. Blessings.