Thats one I cam not too sure on.
IE. does Elo use the mc sliding enchanting rule of thumb or does she use her own scale, but from past experience with vorpal weps it seems to be something like this lvl 0 base wep 10% chance of head then +5% for each lvl there after so lvl4 would be +30% that would seem about right but I have no facts or figures to back it up just past play time, ps its does not effect the drop rate of wither skelly heads luck/looting helps more there, plus 30% does not mean a head every 3rd kill its 30% chance for every kill, you could kill a player 5 times get nothing or get 3 the art of randomness.
ps if some one has facts & figures & I am a little off or way off plz feel free to correct as you see fit, again its just mho bases off game play.