\(\begin{align} &f_x = (1-x)f_0+xf_1\\ &\text{lerp}(x,f_0,f_1) = f_x \end{align}\)
Desmos
\(\begin{align} &x = \dfrac{f_x-f_0}{f_1-f_0}\\ &\text{unlerp(f_x,f_0,f_1) = x \end{align}\)