Scale Sampled Depth Value
Algebraically speaking, it's not too hard to modify $d$ in order to scale $z$. If you want to scale $z$ by a factor of $c$, giving $z' = cz$, then
$$
\begin{aligned}
d' &amp;= a \frac{1}{z'} + b \\
&amp;= a \frac{1}{cz} + b \\
(d' - b)c &amp;= a \frac{1}{z} \\
&amp;= d - b \\
d' &amp;= \frac{d - b}{c} + b
\end{aligned}
$$
In short, to scale $z$ by a factor of $c$, you would take the depth value and subtract $b$, scale by $1/c$, then add $b$ back again.
Note that if you use a projection with reversed Z and an infinite far plane, then $b = 0$, which simplifies this to just $d' = d/c$.