If you want to fully determine the impedance of your power supply it is not a single value. Instead it is a function of frequency as it is the case with capacitors and inductors.
To get a better understanding I have written a tool to simulate the impedance of power planes in combination with capacitors in the past.
I want to give an example:
This is a powerplane-pair with a distance of 0,7 mm between them.

You can see the locations of caps and measurement points.
Here you can see the simulated impedance at the different measurement points.

some insights that can be derived from it
At low frequencies (up to 1 MHz) the 100 µF Elko dominates the impedance. The higher impedance at Zin(i002) is from the resisitivity of the copper planes. Distance to the capacitor is not important.
At higher frequencies (~ 10 MHz) the local decoupling caps (100nF) are effective. Here the distance plays a big role. Zin(i002) has no local 100 nF cap so its impedance is higher.
At even higher frequencies (100 MHz) you can see the effect of the plane inductance. So Zin(i002) shows higher impedance due to the thin connection to the rest of the plane.
At frequencies above 500 MHz (depends on size of planes) board resonances are dominant. You cannot improve the impedance in this region very much with decaps.
You can find the tool at
https://leiterplatte.jimdo.com/pdn-sim/