Well the gemorah explains the mishna, that after 20 amos – ain yoshev btzel suckah ela bitzel difanos. Which means that the reflection from the sun will not hit inside the sukah when the walls are higher than 20 amos. Now you can derive from this that below than 20 amos, the schach will be mitzel (create shade). Now obviously per portionaly, the higher it goes the less you need the schach.
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