To return a bit to the main topic, the best way to judge if a map is suitable as a lowest diff is still actually looking at the map. And in my opinion, any star rating cap for a lowest diff should be lower, rather than higher. Putting it at the highest peak of normal will result in some lowest diffs that are mostly unsuitable as the lowest difficulty in a mapset.
To give examples from my own mapsets, I certainly would not want this map or this map as the lowest difficulty in the mapset, as they are far too hard for a new player but still good as a "Normal" diff.
To give examples from my own mapsets, I certainly would not want this map or this map as the lowest difficulty in the mapset, as they are far too hard for a new player but still good as a "Normal" diff.