Shame you didn't receive a reply to you question as i am also wondering about how to shim the sensor. I have plenty of adjustment on mine so i shouldn't need to shim but i wouldn't mind knowing weather the sensor need to be moved toward the front of the camera body (where lens attatches) or toward the rear of the camera?
You should look up a tutorial on setting up back focus for astrophotography to double check me, but look at the filter section here....
https://optcorp.com/... divide it by 3.
Glass between the last lens in the optical system (telescope or camera lens) and the sensor, effectively reduces the back focus distance of the system because it alters how fast the light is bending. It gets a little confusing, because when you add that glass you have to add a corresponding spacer, which makes it sounds like it is increasing the back focus, but what's its really doing is making the sensor act as if it were mechanically closer...its a semantics thing.
But to summarize it - the back focus of an EF mount camera is about 44mm assuming all the glass is in place over the sensor. If you remove glass there, the light will bend faster and hit its focus point sooner, meaning you need to move the sensor FORWARD toward the flange mount. In the linked example, we added glass to the system and had to move the sensor farther back by adding a spacer. In the example of removing the IR filter on a DSLR, we are taking glass away and therefore need to 'remove a spacer' or in reality shim the sensor closer to the flange.
How much? The math works out to be about 1/3 of the thickness of the glass. This will depend on the exact refractive index of the glass, but its a starting point. For DSLR mods, ideally you'd find a clear glass made of the same material as the original filter, that is the same thickness, and replace the removed filter with that. That, of course, is quite hard to do which is why the sensor gets moved some to make sure the effective back focus is the same and the camera can reach its normal focus range.
I hope that makes sense. There are some good youtube videos on it too if you find a visual explanation better.