School Boy Error - Professional Studies

Well we all make them - that is, School Boy Errors. Mine was the wrong knowledge of ND filters. This came about when using my old Petval Lenses. These lens by nature are fast and  without the aid of a shutter exposures have to be around 2 seconds to manage the timings. However, to get 2 second exposure on a large format lens of around F4, light coming through the lens has to be controlled. This was achieved in the past by a 4 stop ND filter. On  a previous assignment using this technique the negatives always seemed over exposed by around 1 to 2 stop. This was always semi corrected in Lightroom but I wanted to get the exposure correct. I researched and double check the Petzvals Lens aperture along with the filters I was using. This is where an error was highlighted. I had measured and calculated the F.stop of the lens from the stop opening rather than the  back element of the lens. I had thought previously the lens was F3.2, my new calculation put it at F4. The other piece of enlightenment was on the filters. I had been using an 4 Stop filter. I had presumed this actually meant 4 stops of light would be stopped coming through the lens. Oh no, it is actually 2 stops. So this is why my negatives where always between 1 and 2 stops over exposed. An 8 Stop filter was bought, which actually stops 3 stops of light coming through the lens and using F4 on the light meter meant the over exposure was corrected.