I have the same setup (YM30 and SD20). I assume you are aware that there is a second zinc (pencil style) on the port side of the saildrive unit (inside the boat)?
As to how quickly they are used up, so far my experience matches yours. Last season was the first, and we pretty much went through the exterior zinc in about 7 months. FYI, you can get a split version that can be changed underwater. I haven't climbed down into the bilge to check how the pencil style zinc fared yet (on my list for spring commissioning).
Can't answer your original question though (sorry).