I'm not sure I completely understand what you are doing, but I don't think "locking exposure" should be part of your process, because different images require different exposure. The profile simply helps to assure that whatever your exposure, what you see on your colour-managed display should be reproduced in the file generated from your colour-managed scanner, provided the pre-scan shows the impact of the exposure adjustment you make in pre-scan mode. Profiling scanners for the shadow areas can be difficult. I agree with Jonathan's point to avoid clipping highlights and shadows at the scan stage. It is very difficult to match the appearance of a slide and a scan of the slide because there is no luminosity management of the conditions in which you are viewing the slide outside of the scanner, unless you do a lot of work on the profile to get the result to match a very consistent slide viewing condition, such as from a light table whose lighting remains pretty constant. You may wish to consider downloading a demo version of SilverFast Ai6 for that scanner, obtaining one of their scanner profiling targets for the film of most interest to you and use it to generate a custom scanner profile using their Auto-IT8 feature. See whether that helps. It may be useful for you to read a couple of my recent articles on this website which describe this profiling process and its outcomes in some depth.