Quote: 
phi_c = (phi1*Y1*d1 + phi2*Y2*d2 )/ (Y1*d1 + Y2*d2) (1)

This formula is only sensitive to the ratio YL/YH (which I've called E1/E2).
I took the parameters from Penn, chose two fiducial coating thicknesses (a λ/4 + λ/4 coating, and a 3λ/8 + λ/8 coating), and used this (along with Penn's reported values for E_{1}, E_{2}, φ_{1}, and φ_{2}) to compute two fiducial values for φ_{c}. Then I solved these two equations for φ_{1} and φ_{2}, and allowed them to vary parametrically with the ratio E_{1}/E_{2}. 