I don’t know if you’ve seen this post, here @Graham has uploaded some blank frames using 3 different lenses:
I’m not sure that there is a reliable way of quantifying vignetting, as a guide with my setup using a masked CS-Lite and a home brew film holder about 3cms above I photographed the afore-mentioned blank frame of colour negative at my chosen aperture of f5.6/8 on APS-C with a 75mm Apo-Rodagon 2x. I then converted it to B&W and adjusted the exposure slightly to get the centre at 50% in Lightroom. The corners were then around 48.5%, no less than that, barely noticeable to the naked eye and easily corrected. I find that the shape of the vignetting shows up much more clearly on the thumbnails.
You can if you like just have an empty frame and expose accordingly, I like to use a piece of blank colour negative film in case there are any unexpected effects pertaining to the presence of the film itself. If I was photographing transparencies I wouldn’t bother about the radial filter but colour negative conversion boosts the contrast greatly and vignetting shows up as pale corners.