Try this,
Wait for 3 different streets to show
Bet them once (9 numbers)
If hit, restart
If miss, then wait for a further 3 different streets to show
And bet those 6 streets once (18 numbers)
Hit rate very good in testing
Possible flat bet as well
An example. Below are the numbered streets hit taken from actuals
11
6
7. Three different streets. This is the trigger to bet these 3 only once
7. Win +9
2
12
2
12
5. Trigger for 2,12,5
10. Loss, track from here for another 3 different.streets
6
2. <not here because 2 is already included
2
5 <or here
6 < or here
12 <wow or here
7 < here! Bet 2,12,5,10,6,7
10 win +15
8
6
3. Trigger. Bet 8,6,3
7. Loss
7
6
1
10 trigger. Bet 8,6,3,7,1,10
3. Win. +21
3
3
1
7 trigger. Bet 3,1,7
6 loss.
12
6
1
9. Trigger. Bet 3,1,7,12,6,9
10. Loss +12
Etc