First experiment is to see how large a customer base there is. You want to see how often people are looking for a product like yours or your competitors. Make an Adwords campaign with the right keywords, bid high enough to always be shown first, and then run this for a while to get an idea of what the search traffic is like.
Second experiment is to see what portion of that customer base you can capture. So, continuing on from the previous experiment, you now want your site to be set up, and then measure what percentage of those searches result in people actually clicking on your site. And then what percentage of those clicks result in them actually putting in their credit card information and paying for the first month. If you can't have people signing up on their own, then give them a form to submit their information instead, and then make some estimate on what percentage of those will fall through.
Then, that gives you an idea of what your baseline growth is. And then anything beyond that, i'm sure you would've already hired someone who could give you way better industry expertise than you would find by asking HN.