So basically, if I’ve understood this correctly, your glass scope gets zeroed & puts the reticle into position mechanically so that the bullet hits what you see in real life.
Then you place a clip on thermal on the front which places a small flat screen in front of the glass. The scopes reticle is then seen in front of whatever picture is shown on that screen, which means that the target on the screen must then be perfectly in front of the reticle. If it is then the bullet will hit where the reticle is seen. Correct
‘Zeroing’ the reticle is basically moving the image seen on the screen so that it is correctly aligned with the reticle in the scope. Yes
Now provided you can remove & refit the add on with absolute repeatability the image seen will always be in the correct place. Yes
However, if the add on gets put back on slightly skewed to one side or the other the image will not be in the correct place. So, for example, if the add on is skewed slightly to the left then the reticle will sit off to the right of the centre of the image. Yes
This is where basic trig comes in as a very small error between the alignment of the add on which is maybe 6” ahead of the reticle will translate to a very significant error between where the reticle is placed on the image compared to where the bullet lands at 100 yards.
I can now understand why there would be an ‘index’ mark available on the add on screen that you can check the unit is properly aligned on the scope each time it’s used. Not necessary