CreateFolder.cs

public static void CreateFolders(Job job)
{
    var path = "J:\\" +
               JobFolderRange(job.JobNumber, 1000) + "\\" + 
               job.JobNumber + " " +
               job.Client + " " +
               job.Address;
    CreateDirectory(path);
}

public static string JobFolderRange(int jobNumber, int rounding)
{
    var lower = ((jobNumber / rounding) * rounding);
    var upper = ((jobNumber) / rounding) * rounding + rounding;

    if (jobNumber == lower)
    {
        lower -= rounding;
        upper -= rounding;
    }

    lower++;

    return lower + " - " + upper;
}