Let's assume a multi-layered stack of - 

(Bottom) Layer 1: n (@ 633 nm) = 1.21 / thickness = 60 nm

(Mid) Layer 2 : n (@ 633 nm) = 1.47 / thickness = 30 nm

(Top) Layer 3 : n (@ 633 nm) = 1.48 / thickness = 10 nm

How can I calculate the effective refractive index of the stack @ 633 nm?

Can I use the simple EMA formula of "1/n_eff = (1-phi)/n_c + phi/n_d"?