The average EOR of 678 is 1.3% and the initial house edge is 2.6%. There are 12 sixes, sevens and eights per deck representing a ration of 10;3 of non 678 to 678. So we should expect to see 3 678s per quarter. Except this is for the 15:1 suited 21 table. The 10:1 table increases the house edge to 5%, so you are going to need on average 4 excess 678s per deck to break even. So at .5 deck dealt you would need 6 extra 678s to break even. You would expect to see 6 of these at this point so you are looking at a strike point of 12 678s at .5 deck.
How did I do?