I don't know what your ROR is there, but it is not 50%. +6 is very late for the max, certainly not optimal for most shoe games. Best is usually +3 for poor to mediocre pen, and +4 to +5 for better pen. This will increase your win rate, lower your long run index, but also increase your risk. But ROR should still be well below 50%.
Now, you added "only positive games". Does this mean that you will play only positive ev counts? If so, your risk is substantially less and you don't need a 1-12 spread. A 1-4 spread is more than sufficient and a bigger spread size (assuming the same max) will not show much gain in favorablility.
If there is an advantage in any game, no matter what size, shouldnt there always be a better chance of doubling than losing it all? I am no math expert but it only makes sense.
correct me if i am wrong???
For certain games, for certain (small) bank sizes, there is a bigger chance of losing it all than doubling the bankroll.
For example, consider playing 25c full-pay deuces wild (100.7% return). With a bankroll of a few hundred dollars, you are very likely to lose it all before doubling it, because the big paybacks come from rare hands (royal flush and 4 deuces). I don't have the precise numbers in front of me, but I recall that you need about a $3000 bankroll to achieve a 5-10% risk of ruin.
which is what you get if you bet it all on one hand. As tora-papa points out, there are some situations where you odds may be less than 50:50. But in bj it is nearly impossible for a skilled player to have less than a 50% chance of doubling before going broke.
In my opinion, discussing the chance of doubling a bankroll before losing it should refer to play with some optimal or reasonably-close-to-optimal betting strategy.
Betting the entire bankroll on one hand is obviously not an optimal betting strategy. It gives you a lower bound for the chance of doubling your bankroll, assuming that doubling your bankroll is your only objective. This is the case where you have $500, need $1000 to pay your rent tomorrow, and amounts under $1000 are completely useless. But people have more rational utility functions, and I'd rather have $1200, say, than $1000. My betting strategy is intended to increase my wealth, not simply to double it at any cost.
I certainly agree that with an advantage and a reasonably sized bankroll, playing BJ, the chance of doubling before losing it all is over 50%. But some very small bankrolls doubtless have under 50% chances of doubling. I don't know where the line is.