Answer: 45,5mm is the value which we have for this Barlow in our design - so we did a quick-and-dirty-test of the magnification.
To do so, we used a finderscope which gives enough field of view to count the shingles on the next roof. We placed a small camera in the eyepiece clamp of the Barlow lens, so that the camera is as close as possible to the position of the field stop of the Classic Ortho/Plössl eyepieces, i.e. 2mm inside of the body.
Then I took one photo with the Barlow element removed from the body:
and another shot with the body; the camera now had to be ca. 22mm farther away from the finderscope:
Don't be too critical of the sharpness - it was manual focus on a shaky tabletop tripod, but it is good enough to get a measurement of the width of two shingles.
The expected magnification of 2,25x was met pretty good - especially if we keep in mind that the sensor position was not marked on the outside of its body, so its position was more or less guesswork. But the camera sensor was aproximately 55mm away from the part of the barlow element, where the thread ends. Also, it was just a quick measurement of the shingles in Photoshop, nothing of high precision.
So, if we place the camera at the same position as the field stop of a Classic Plössl, the magnification is as expected.
If we take the formula
magnification = 1- (working distance/focal length)
I get 2,25x=1-(55mm/focal length); or a focal length of 44mm. This is not too bad for the measuring error and an expected result of 45.5mm.
Could it be that you placed the camera sensor in another distance? If you push it deeper into the eyepiece clamp of the Barlow, then we get closer to your 35mm.
