An albino bird lacks all pigments while a leucistic bird has a reduced intensity of pigmentation. If a bird has a reduced intensity of pigmentation over part of its body, then it is leucistic. If a bird has reduced intensity of pigmentation over most of its body, then it is still leucistic. In summary, any reduced intensity of pigmentation over part of a bird, whether major or minor, would make that bird a leucistic bird. Therefore, to say a bird is "partial leucistic" is incorrect. The correct description would be to say a bird has major or minor reduced intensity of pigmentation.
You wouldn't have a bird that is "partially pregnant" no more than you would have a bird that is a "partial leucistic." A bird is either leucistic or not leucistic. You can however, have a bird who has more or less areas effected by leucism.
Also, leucism can very well affect more than feathers. So to say leucism is a lack of pigmentation in the feathers is incorrect. I have seen birds that have lack of pigmentation in legs and beaks as well. A bird that has lack of pigmentation in legs, beaks, etc., is not a "partial albino," but a "leucistic bird."
As always, I do enjoy reading the replies from the good people at the BirdForum. I wish everyone the best of luck in their birding ventures this year.