someone wrote up a tutorial covering what i think you’re wanting to do, though i think i would do steps 2 & 3 differently. (i’m sort of indifferent to accuracy values myself – so i’ve never actually tried doing this whole process myself – but it looks like it’ll reasonably do what it claims to do.)
as far as why you might have “missing” accuracy values, you might want to look at the link i referenced in an earlier post in this thread, and see if that sounds similar to your process.