var chunk = 100;
var s = "long-long string with some n chars";
var messages = new List<string>();
for (var i = chunk;; i += chunk)
{
    if (i >= s.Length)
    {
        messages.Add(s.Substring(i + 1 - chunk));
        break;
    }

    i = s.LastIndexOf('n', i >= s.Length ? s.Length - 1 : i, 200);
    if (i <= chunk)
    {
        messages.Add(s.Substring(0, i));
        continue;
    }

    var len = messages.Last().Length;
    messages.Add(s.Substring(len, i - len));
}