Ask HN: Want to test with users for potential feature, how to measure success?

I want to run an experiment with a user cohort, basically telling them that we've developed some feature and order them to 'unlock/activate' it by hitting a button in the email. There is something that I can't seem to figure out; at what percentage of opens would this be a success? I imagine emails convert less than just putting it in someone's face. Any suggestions in the community?

Edit: sorry for crappy grammar in the title, had to fit it into 80 chars