I personally used 30 back when I played mania as my main mode but as of now I find using 29 much more readable as I have not adjusted to reading as fast as I could previously. I'd just recommend using what you feel to be comfortable and easier to read patterns with depending on what skin your're using since its mostly up to preference really. For skins with a lower hit position, I find fast scroll speeds to be more needed. For skins with a higher hit position, I find lower scroll speeds to be more needed. There isn't really a "good" speed that should be used by everyone imo. If you feel as 28 to be okay and have decent accuracy, then stick with it and try to improve from there (reading patterns/ other skills). Then bump up the speed maybe when you feel like you can read faster if you want