Really, if you accept that the DRS as it is is there to overturn "howlers" then Clarke's thought process before reviewing an LBW should have been:
1) Did I hit the cover off it
2) Did it hit me a foot outside off
3) Did it pitch a foot outside leg
4) Did it hit me on the thigh when i was halfway down the pitch
If the answer to all the above is no the Clarke's conclusion should have been "Accept the decision, it's too close to call and we might need the review for a genuine howler later in the innings" as opposed to "What would Watto do..."