I did see that calculator. However, there are also articles online that state it's based on your adjusted gross income. This is only done once per year on your tax return.
Perhaps you pay based on your AGI but then at the end of the year you would enter your insurance cost on your new tax return. It would then be adjusted based on your current year income so you would get a refund or pay more based on whether you paid too much or not enough.
I think this would seem to make sense for people who have variable incomes such a someone who is self employed. It may be difficult to predict your income for a home business for example. Therefore, adjustments at the end of the year tax return seem to make sense. I just can't seem to find any conclusive answers anywhere online.