thank you, I'm going to try it then.
yes I'm aware about that shutter speed wont affect flash exposure. Is your concern that without shutter affecting exposure I may not have enough aperture steps to perform the test? Valid point, good thing about the test method I'm trying to use is that you photograph stouffer step wedge using the same aperture/shutter. 31-steps of the step tablet will represent 31 exposures within one frame.
Details are here (more is in the book)