"A wither skeleton has a 2.5% chance of dropping a wither skeleton skull when killed by a player or tamed wolf." So IIRC, you do 0.0250.0250.025 to get the chances of it and it basically is 0.000015625 (~0.0015625%)

Only 1 attempt of 64000 attempts is a perfect. WOW. I think it is a Little bit insane to try to hace that luck; I mean, in the time that you need normaly to have the luck, you can capture normaly like 16 shinys in Pokémon X or Y jajajaja XD

To be in mind, the normal would be going to be to need to kill like 120 Wither Skeletons, so dont be angry if you dont have the luck to a WR lol :V XD. Me I am happy if I do a run that I less than 100 Wither Skeletons lol XD