My problem with the fractions/percentages is that they are trying to represent 100% of a bird...all genes involved. I understand we talk about a bird being 1/2, 3/4 Ameraucana or whatever, but all individual traits/chacteristics aren't created equal. For some individual traits (e.g., muffs/beard) you may only need a couple generations to be "pure" for them. For other traits, especially those that may involve extra unknown modifying genes (e.g., egg color), you may not be at 100% for that trait even after 10 generations.
Of course this is just my opinion and I'm not saying your plan of attack is wrong. As you look to lose the Orpington "look", try to find Ameraucana breeders (not necessarily show quality), that may "over compensate" to get you to your goal quicker.