I have 2 miners, 1 6850@262kj/s and 241WorkUnits/m and 1.5% stales; and 2 7950 @ 1230Kj/s and 1154WU/m with 1.7% stales. There are no reported HW errors.
I'm not a mining expert, but if I understand right you're staling for the whole setup is currently sitting at 3.2% (which is not measured in the calculations), plus whatever % you're donating (1% default), that puts you at 4.2% less LTC than the calculations. Also allow some 20-30 kh/s fluctuation 1.3%-2% making it a total of 5.7% on average (3.762 LTC/Day).
Still higher than what you're currently getting (if you're 24/7 mining without any drops)
Edit: Did I just forget the 2.5% fee? Sure did...
That puts you at 8.2%, which is 3.662 LTC/day
I think you're doing just fine :-)