Hi,
I would calculate average stock in pcs, if I run MC44 with period of inventory turnover 01.05.2013-11.06.2013 and no restriction radio button selected, I obtain as average stock value 7,738.
By MB51 I can see for a material the following:
TEST 101 5000137977 11.06.2013 1 PC
TEST 201 4900616004 07.05.2013 1- PC
TEST 101 5000137971 07.05.2013 10 PC
Clicking on the "detailed display" button I can see:
Posting date GR GI Stock level
07.05.2013 10,000 1,000 9,000
11.06.2013 11,000 1,000 10,000
Over the net I found the formula of the average stock in pcs that is (initial stock+final stock)/2 but the result is different from the above 
7,738
Thanks
Best regards