It's very important to realize that the payout percentage expresses expected payout from a single game round. Let's assume that you bet 100€ on roulette and win 1200€. Then you continue playing and stake 12 times 100€ = 1200€. The payout ratio (RTP) must be used on each game round separately. The expected statistical casino profit in this case would be (100€ + 1200€) * (100% - 97.3 %) = 1300€ * 2.7 % = 35.1€.
Note that 97.3% is the RTP of European roulette. If you continue playing with your previous winnings, then you should expect to lose more than the previously declared house edge of the game. Most of the players place bets from their previous winnings again and again, losing a portion of their money each time. You'll have an average RTP of 96% when you play live Bitcoin games (Evolution, Ezugi, Pragmatic Play Live or so).
If you want to be a smart player, you must distinguish between the RTP of the game (which applies only to a single game round) and the expected return of your betting system.
Your betting system is determined by how you play during your whole stay in a casino or your entire session in an online casino. This includes selection of the game, its variant and settings, sizes of your bets and decisions about when to stop playing.
It's difficult to imagine the RTP of a betting system taking into account just one player. That's why we always run simulations with at least a million of players to get reliable results. The expected return of a betting system can be defined as a ratio of the overall winnings of players who managed to hit their "target", to net losses of players that weren't that lucky.
Be aware that some websites swap these two definitions. Their definition of game RTP might give a false impression that players statistically lose only a small fraction of their money, equal to house edge of the game. The rest of the money should then be redistributed among winners (other players using the same strategy that managed to win).
In reality, if players place bets using their previous winnings, they lose more. The RTP of a poor betting system may drop far below 50%. Even for blackjack – a game with an RTP of 99.5%. As you will see, the RTP of bad betting systems is close to 0%.
The optimal betting system is one which has the same RTP as the game played. To achieve this, you must avoid placing bets using the money you've previously won. Theoretically, the easiest way to achieve this is to stake your whole budget in one round. Then keep betting all-in until you lose or win a satisfactory amount of money. Roulette is a very good game for this system as you can choose the odds of your bet.
Let's assume that you have 100€ to play with and ending up with 900€ will satisfy you. Placing 100€ on a square (4 numbers) is almost the optimal strategy in this case. You will either leave the casino with a nice amount of money or without 100€, but your chances are quite fair. The biggest disadvantage of this approach is that you will play only for a very short period of time.