def _nzt(bt, zt, bt1, bt2):
    """Evaluate appropriate z-location for countersunk hole."""
    if (bt - 0.5) < zt <= 0.5:
        zt1 = bt1 - 0.5 + (zt - bt + 0.5) * (1.0 - bt1) / (1.0 - bt)
        zt2 = bt2 - 0.5 + (zt - bt + 0.5) * (1.0 - bt2) / (1.0 - bt)
    else:
        zt1 = bt1 - 0.5 + (zt - bt + 0.5) * bt1 / bt
        zt2 = bt2 - 0.5 + (zt - bt + 0.5) * bt2 / bt
    return zt1, zt2