Interesting. I thought the 26x were measured from, the film plane; that would make the enlargement factor 25. A nice (round) figure making calculations in your head much easier than 26x. (With 25x just multiply by 100 and divide twice by two.)
I have an old test chart that needs to be photographed 40x, not 41x, from lens center, which prompts my remark here.
Have a nice day.