Just did some calculations and it looks like back dividends are something close to:
0.00034120 per Mining or Purchase
0.00200077 per Selling or Purchase
As of October 25th there were 163054 "Effective Units" outstanding. That means DMS is about 382 BTC behind on dividend payments.
I finally got around to updating my spreadsheet.
Amount of unpaid dividends as of July 18th, 2014:
0.00032901 Per MINING or PURCHASE
0.00216280 Per SELLING or PURCHASE
Effective Units 163054
Total back dividends owed: 406.29997924 Bitcoin