Complex sentence: The flag assembly was stored immediately behind the left side of the LM ladder, where temperatures were calculated to reach due to the descent engine exhaust for most of the landing, but to climb as high as during the final 13 seconds at touchdown.
Split into simpler sentences: The flag assembly was stored immediately behind the left side of the LM ladder. Due to the exhaust from the descent engine, the temperatures were calculated to be for most of the landing, however the temperatures would increase to during the final 13 seconds at touchdown.