With sprockets you can have any number of teeth from 16 to 24 Tom, and I think 19 is a good compromise for the Pro Connect to retain good range.
It's normal for the new gear cable to stretch slightly initially, and it probably only needs the slightest tweak. Where the cable outer leaves the handlebar changer there's a screw out socket that the outer sits in. Just screw it out a touch and test at that. It's easily tweaked on the road to get it just right.
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