[robski]

I would go along with the 0.88 figure.

Crop factor has nothing to do with magnification. It is related to the field of view (FOV) a lens will give you for a given sensor size. In a nutshell it is saying the FOV is equivalent to using a 1.6 x longer focal length lens on a 35mm frame camera.

If the magnification was 3 then a 7mm object would fill the frame width on a 400D.

As a rule lens magnification calculation formulae are prone to error if they purely rely on the lens focal length and do not take into account a focus distance or native lens magnification. The main problem being (especially for telephoto lens) the stated focal length is not constant though out the focus distance range. In this case it maybe something like 135mm at infinity and reduce to something like 100mm at the closest focus distance.

For anybody who is interested I have a M42 screw thread 90mm Macro lens which I not longer have any use for. I bought it new for my Practika film camera some 35 years ago (only bit of kit I have left from that era). Missing front cap but otherwise OK. PM me if interested.