To meet this target impedance at high frequencies, decoupling capacitors must provide a lower total ESL and ESR.
The PDN impedance must be kept below this value from DC up to the maximum frequency of interest to prevent voltage droop during switching transients.
Target Impedance (Z_target):
$$ Z_{target} = \frac{V_{dd} \times \%Ripple}{\Delta I} $$
Where $\Delta I$ is the transient current step (often assumed as 50% of $I_{max}$).