the toolkit has an aweful lot of c++ code in it if you havent been thru it already:
https://www.twain.org/devfiles/twainkit.exe
the problems I’d seen/had with setting resolution in the past we mostly due to the fix32 structure that you use. Take a look thru how it is accomplished in the toolkit and if you’re still having trouble open a new thread with the issue.
cheers.