Constant ETA_REFERENCE 

pub const ETA_REFERENCE: f64 = 0.69;

Fig. 6’s η at M_n = 0: 0.69, midway between its square and diamond there, about 0.68 and 0.70, which Jorgensen takes from Fig. 4 for its two bodies of fineness 10 and 12 (this module’s own reading of Fig. 4, ETA_BY_FINENESS, gives 0.685 and 0.701).