Thank you for the feedback, a calculator is a good suggestion! We realize there are a fair number of people that will be turned away by this, we'll see what we can do for a better middle ground.
I’d also suggest you have a free plan for very small teams. You can already see how many slack members they have. Make your tool so people just make it their default and then as the team grows the naturally start paying you up the tiers.
Actually we did offer a free plan for up to 5 users and even 10 at one point. What we've found is the collaboration overhead during incidents at companies at that scale wasn't too useful and pivoted away. Instead we offer a 14-day trial so customers can get their feet wet without contacting us: https://rootly.com/users/sign_up.