In this daf. The reason for the maximum hight of the קןרה is to function as a reminder. (Unlike Rav who learnt it must be like the ENTRANCEWAY of the sanctuary). This is like the Schach of a succah. Must the קורה be within 20 amot or can the gap be 20. 2 types of amot 5 tefach and six tefach, you must always measure strictly.