Hi George
Did you get any further with this? I just got a DP2 which under-reads about 1.5 stops above EV 10 (100ASA) which I can easily adjust by changing the film speed. However under EV10 is gets increasingly erratic to the point where it is out by 3 stops or more. I will only risk taking it apart if there are some easy to find adjusters inside. I have a working DP1 but I find the DP2 seems to have a cleaner and brighter prism - but maybe the DP1 is a bit dirty.
Thanks