There is a direct correlation between a form template URN and the my namespace uri that is required by infopath forms referencing the template.
This little method will give you the namespace required for a given template URN. This is nothing complex, but is handy nontheless.
public static string GetInfoPathNamespaceForUrn(string urn)
{
string inputdate = new Regex(".*myXSD-(?<date>.*)").Match(urn).Groups["date"].Value;
string outputdate = DateTime.ParseExact(inputdate, "yyyy-MM-ddTHH-mm-ss", CultureInfo.InvariantCulture).ToString("yyyy-MM-ddTHH:mm:ss");
return string.Format(@"http://schemas.microsoft.com/office/infopath/2003/myXSD/{0}", outputdate);
}