This is too advanced for the calculator alone. It requires manual calculations. The calculator doesn't use monitor size for anything AFAIK. It just looks at the fov values.
Here is an example of two monitors (one is twice the size) set to 90 field of view. The calculator will think that these two scenarios are the exact same thing, as it just looks at the 90 field of view, when in reality they are very different.
Here are two examples where the field of view is adjusted to result in the same projection. First image syncs to the large monitor, second syncs to the small monitor.
You want to match the projection, and make the field of view dependent on the size and shape of the aperture (the hole that you view the game world through, the monitor basically). In the first case, the small monitor is adjusted to 53.13 fov, and in the second case, the large monitor is adjusted to 126.87 fov.
On your current monitor, switch between ~53 fov, 90 fov, and ~127 fov, and you will quickly notice that the literal sensitivity of the mouse will be affected. You can't just keep the same cm/rev (distance for 1 revolution, aka 360 distance). The projection has to scale to accommodate the desired field of view, which zooms the game in/out. This is because the monitor doesn't magically grow or shrink to accommodate the desired field of view.
You have to scale the rotation with the projection. The cm/rev needs to scale at the same rate as the change in projection.
You can use 0% MDV to scale by the relative change, as the monitor size is not required when it is constant. However when converting between different sized monitors, you have to do manual calculations.
To match Shroud's settings, you have to use the same cm/rev as him, and have the same field of view within the 23" portion of his monitor. This would be 100.4629 fov. Since you want a different field of view, you have to scale the cm/rev by the change in projection, which is where the 1.5 -> 1.25 comes in.
Convert with 0% MDV if you are just working relative to your own setup. 0% from Overwatch to Apex Legends. That will preserve the literal sensitivity of the mouse. This isn't always desirable though, as the required movements to accomplish tasks depends on the game and how zoomed in your are. For example, Fortnite is really zoomed in, and requires a lot of mobility and rotations for core gameplay. It would be more beneficial to just use an entirely different sensitivity, instead of converting with 0% MDV. A new sensitivity generated by using a different method, which preserves a desired property, or just picking a new one arbitrarily.