Assassin's SP Build?

Should I follow this build?
If not, please recommend me one.

3 Mastery
Max Critical
2 Mastery (total 5)
4 Booster
Max Haste
3 Endure
15 Drain
Max Mastery
Max Drain
14 Booster (total 18)

Why does Drain come before Mastery?

Also, I have maxed Nimble Body so which should I take the SP away from? Drain or Booster? Or combination of the two? Please tell me pros/cons.

THANKS! http://forums.mapletip.com/index.php?showtopic=114840