Why a previous month requirement is a step
A single won short and a whole month of rewards disappears. Here is that structure written out.
A reward rate is continuous. Spend a hundred thousand and you earn on a hundred thousand, spend two hundred and you earn on two hundred. A previous month requirement is not like that. It asks only whether you passed a fixed amount, and if the answer is no the whole month is gone.
A function that jumps instead of rising smoothly is a step. That one step is what turns a several card problem from something you can sort into something where combinations have to be tried.
Excluded spending twists it once more. If the row with the best rate happens to be the row that does not count toward the requirement, you can pour that row into the card and still fall short, ending with nothing. In that case some other spending has to be left on the card to carry the requirement.
The calculator handles both at once. Mark which rows are excluded and it separates the money that carries the requirement from the money that earns.
The window the requirement is measured over also differs. The first to the last day of the previous month is common, but some cards count a month back from your billing date, which changes whether the days around the turn of the month count. This page treats a month as one block, so for a card with a different window put in the spending for that window.
A card with several tiers gets one field here for now. Put in the tier you intend to clear this month, and run it again with another value if you want to see that one. Handling several tiers at once multiplies the combinations to try, so it is not in yet.