def calc_gamma_ref_via_menq_2003(c_u, p_eff, p_atm=101.0e3):
    """
    Calculate the reference strain for a modulus reduction curve from Menq

    :param c_u: float
        Uniformity coefficient (Grain size ratio d_60 / d_10)
    :param p_eff:
    :return:
    """
    big_a_gamma = 0.12 * c_u ** -0.6
    n_gamma = 0.5 * c_u ** -0.15

    return big_a_gamma * (p_eff / p_atm) ** n_gamma / 100