There is no percentage for getting an egg move. When you pass down egg moves the Egg will know them when they hatch. Guaranteed.
For more on egg moves and breeding: http://pokemondb.net/mechanics/breeding
Edit: Upon re-reading the question I am guessing you think that when eggs hatch they randomly have egg moves? That is not how it works. To get an egg move the Male Pokemon has to know these moves then breed with a female(that does not need any specific moves) then the egg will know the egg moves that it's father passed down.