Thanks for your response. Its the latter effect I am interested in. "My target brightness is in-between existing steps etc." To the best of my recollection, the post I saw originally on the subject intimated that there was a native to iMac method of fine tuning. But maybe I misread, or merely had rose tinted specs on at the time.
I'm using a Spyder3Elite which only compares current brightness to the target; leaving me to match them via the keyboard. Hence my interest. Again many thanks.
Cordially,