WestJet boards back to front, so Zone 2 is for the rear seats. I can tell you that Row 19 was a Zone 2 a couple of years ago, and Row 16 was a Zone 3. I think the transition may be somewhere around Row 17, but I cannot say for certain.

Emergency exit rows are like zone 1.5, after premium, status, and pre-boarding, so you may just wish to do that if you're THAT worried.