If you play all your games with black bar at the same aspect ratio you don't need to account for this.
However if you play some with black bars and others in the full aspect ratio of the monitor you need to account for the black bars in the calculation.
For instance converting MDH 100% from full aspect ratio to black bars. If you want to keep a consistent movement on the monitor you would not convert the movement to edge of the full aspect ratio to the movement to the edge of the black bar (where the image ends). You still want to convert the movement to the edge of the monitor, which means accounting for the black bars.
When stretching the horizontal FOV takes up the entire monitor, so the calculations will be correct although vertical sensitivity will be different.
As an example, CSGO with black bars should use the monitor resolution because the only thing that changes is that the game replaces some of the image on the edges with black bars. Everything basically stays the same, you just see less:
However when you stretch you have an FOV of 90 covering the whole monitor, so you need to enter the resolution you play with, and calculate a new sensitivity (if not using 360 distance, which will stay the same regardless):