Three rows laying sideways packed tightly(each of these hold roughly 1000 cards) followed by stuffing three rows on the same level (roughly 150 cards each)to fill in the space. This is followed by three more rows of 1000 cards crossed over and then 3 more 150 card rows. After that the remaining space on top I put 8 sections of about 50 cards each. This should make it total of about 7300 cards in a box.
My boxes are so tight that if I don't tape around the outside of them well, they bust during shipment.