The first question is..... what regulations are you operating under?
What is limiting the takeoff weight to 20,248 kgs? If it is an obstacle in the takeoff path, then you dont have to use this weight, nor if its field limited!
What you need to check is the landing field, the approach climb and the landing climb charts.
All presuming that for Indonesia, the regs are closer to FAA than JAA.
Mutt