Very good point. I like how this could go higher or have in-between values quite easily. In retrospect an equation like this makes much more sense than an intuitive guess, what I wrote down was mostly to use as a start.

I'm not sure if this is exactly the perfect equation for this, given that I think I'd probably want them to be a bit more spaced out if they went to 10 (going further in confidence past 99.9% perhaps) ~i1.