Hi Mike
I just tried this on mine and it works as you want, so it is possible. Before I delve into any of the settings (it just worked so I'd have to dig a bit) can I check that after you set up the grade scale you did set that as the scale for each of the assignments?
Also have you seen the criteria reference block I created for this kind of thing: http://moodle.org/mod/forum/discuss.php?d=96509
or the one created by Mike Tyers for outcomes: http://moodle.org/mod/forum/discuss.php?d=101953