Ratio of the growth rate σ_r to the analytical formula σ_estim ≡ max(σ_SMRI, σ_MGSF), which combines the predictions for SMRI (Eq. (11)) and MGSF (Eq. (17)). The flow parameters correspond to those of Fig. 4 (top panel), with Λ_Z = 10⁵. The white dotted lines delimit the different instability domains (see Appendix C and Fig. C.1 for details). The black dashed line indicate the sufficient MGSF stability criterion (Eq. (15)). The grey dotted line correspond to the combination of the necessary criteria (Eq. (C.12)). See Appendix C.3 for details.