The following system is sofar the more solid IMHO and be sure that I have tested "EVERYTHING"!
As well known the perfect equilibrium in a given roulette number spins is the exception;much more frequent the imbalance.
For instance in 20 spins the perfect balance(10 Red vs 10 Black) has only 17,6% probability to happen.
This statement is confirmed by a math law named "arcsine",but I don't want not complicate the problem.
Since a few years I play a "closed cycle" of 25 spins with the aim to meet the most probable imbalance between two even chances,that is 4!
I wait for 10 spins and when the imbalance is 3, I bet that it becomes 4.If it's goes back to 2,no bet.
Of course after 25 spins(or before if +) stop betting.
I play flat bet or with the progression 1 1 1 2 2 2 4 4 4.
Of course the goal of the system is + 2 unit(high value) per session.
The same system,more resistant,but boring,can be played on a cycle of 100 spins(imbalance
after 50 spins of charting.
When the imbalance is 5,the attack starts.