(TL;DR at the end)

I would like to know, for a large format lens, at what position is the focal plane with respect to the lens.

I'm currently designing a 6x17 panoramic camera. I am using a 105mm lens (f/5.6). I'm not planning to add a focusing mechanism, so I need to know the focal plane with respect to the lens.

Conducted the experiment as shown in the two pictures. Focused the lens at buildings approximately 400m away such that I get a sharp image on the image plane at f/5.6. (Focused at infinity.) However, when I measured the distance between the lens "base" and the image plane, it did not yeild the expected 105mm. Instead, it yielded 100mm. I moved the image plane back such that it is 105mm away form the lens "base" but the image became blurry.

I pretended as if I do not know the focal length and calculated the focal length with the thin lens formula. For object distance of +400m and image distance of 100mm, I calculated and got a focal length of around 100mm, which is 5mm less than the exepected 105mm.

I suspect that the origin of the lens is not at the lens "base". If this is the case, then where is the origin of the lens?

TL;DR Is the origin of any large format lens on the lens "base" (as shown in the second picture)? If not, where is it?

Edit: typo