Not something I can fix. The settings use the open framework’s method of pushing it into the menu, and that doesn’t support min/max settings on the input fields (we could put in a feature request on that script of course). I offer the tooltip that the value is restricted to 2-7, which is enforced by the coding. So yes, you can enter -2, it will treat it as the minimum 2.
The reason 2 is the minimum is because after you finish the reviews for the day, the forecast should at least be showing you tomorrow’s reviews, which wouldn’t happen if it was limited to 1 day (the limit isn’t 24 hours in advance, but just the nr of day blocks the forecast shows). A limit of 1 will just show the empty current day - not much of a forecast.