Well, with a negative edge it's better to flat bet. Imagine at the end of your roulette play, flat bet you'd have bet for example 100K where you'll lose around 2.7% of it. If you use a progression (negative or positive), you'll bet much more than 100K so you'll lose more. At the same time during your play, a positive progression can give you at a moment a lot of money, much more than if you were flat betting. Here's another idea, let's say to deserve 1 unit a number has to come out 2 times. When it came out 3 times put 2 units on it, 4 times put 3 units etc etc till you're happy with the winning.