But a simple screenshot image would not work for all cases, because the calculator page renders differently on mobile devices. On the other hand, if I offer a full working version without sign-up, there would be no reason to sign up. If I show a "frozen" version of the full calculator, normal users might get the wrong idea from that as well.
It seems that you want to prevent people from experiencing your site without signing up first. Why is this? If you hide the value of your site behind the sign-up link, hardly anybody will bother signing up.
With a site like this, I would suggest enabling all features for anonymous users, except the ability to save recipes. Users can play around with the site and make their own recipe. The natural step after creating a test recipe would be to save it - this would be the best place to prompt for signup. I'd also recommend changing your on-boarding flow so that I do not need to confirm my email address immediately.
The end goal is to get enough people interested and using the site, then introduce premium features based on the feedback they give.