I had a similar experience with mine - always read off by 2% of what I
expected, based on the sugar I started with. So I gave up on the
vinometer, and if I've made a dry wine, I just figure from my starting
Brix level what the alcohol level should be. Even if it's not bone
dry, those small graduations on the vinometer will give me more error
than my hydrometer.

But the way you're describing, that's the way I used mine.

Rob