still can't measure share of voice - what am i missing?

what's your definition of share of voice, though? that's usually where it goes wrong. For organic it normally means: take a fixed list of keywords, weight each by search volume and the estimated click share of your position, and compare your total to competitors. Most rank trackers calculate a visibility score like this. The key is keeping the keyword list fixed, otherwise the number moves for reasons that have nothing to do with you.