A phenomenological spin-fluctuation analysis (Ref. 1), based upon inelastic neutron scattering (INS) and angular resolved photoemission spectroscopy (ARPES) data for ${\rm YBCO}_{6.6}(T_c=61K)$, is used to calculate the functional derivative of the d-wave eigenvalue $λ_d$ of the linearized gap equation with respect to the imaginary part of the spin susceptibility $χ''(q,ω)$ at 70K. For temperatures near $T_c$, the variation of $T_c$ with respect to $χ''(q,ω)$ is proportional to this functional derivative. Based on this, we discuss how different parts of the $q$ and $ω$ dependent spin-fluctuation spectrum of YBCO$_{6.6}$ contribute to $T_c$.
