  Following the formalism of the Classical Nucleation Theory beyond the dilute
solution approximation, this paper considers a difference between the actual
solute supersaturation (given by the present-to-saturated solute activity
ratio) and the nominal supersaturation (given by the present-to-saturated
solute concentration ratio) due to formation of subcritical transient solute
clusters, called heterophase fluctuations. Based on their distribution
function, we introduce an algebraic equation of supersaturation that couples
the nominal supersaturation of a binary metastable solution with its actual
supersaturation and a function of the specific interface energy and
temperature. The applicability of this approach is validated by comparison to
simulation data [E. Clouet et al., Phys. Rev. B \textbf{69}, 064109 (2004)] on
nucleation of Al$_{3}$Zr and Al$_{3}$Sc in model binary Al alloys.
