  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 $\lambda_d$ of the linearized
gap equation with respect to the imaginary part of the spin susceptibility
$\chi''(q,\omega)$ at 70K. For temperatures near $T_c$, the variation of $T_c$
with respect to $\chi''(q,\omega)$ is proportional to this functional
derivative. Based on this, we discuss how different parts of the $q$ and
$\omega$ dependent spin-fluctuation spectrum of YBCO$_{6.6}$ contribute to
$T_c$.
