You work that out from various statistical parameters relating to your proven edge.
This highly recommended, introductory book explains in simple terms the details of how to that: Profitability & Systematic Trading, by Michael Harris (Wiley, 2007).
The one thing that's an objective certainty is that if whatever method or system you're using doesn't already have a reliable, statistically significant, proven edge at level stakes, then you certainly can't imbue it with one by varying the stakes according to the outcome of the previous spin/trade/test/selection - and that's equally true of both roulette and forex (or any other) trading.
In fact, I think you're understating that, if anything: the reality is that even in order to learn to understand any of those basic fundamentals, let alone to begin to apply them successfully, you'll need to start by giving up the beliefs currently residing in the space where you'd store, digest and interpet that understanding, otherwise it will simply conflict with what's already there and therefore not be appropriately processed. The book I've mentioned above will help you to do that (among other things). Good luck.