How do you decide how much of your capital to risk on a single idea?

Position sizing is one of those topics that gets less attention than entries, but it often matters more to how a learning trader's journey goes. A common starting framework is to decide, before looking at any chart, the maximum percentage of your account you are willing to lose on one idea. For example, using made-up numbers: an account of 5,000 with a limit of 1 percent risk means you accept losing 50 if your stop is hit. The size of the position then follows from the distance between your entry and your stop, not from how confident you feel. Many beginners find that the hard part is not the arithmetic but the habit. It is easy to size up after a good run of ideas, or to size down so little that a win barely matters. Writing the rule down in advance, and reviewing it on a schedule, helps separate the rule from the mood of the moment. I am curious how others here think about this in practice. When you set a risk limit, is it a fixed percentage, a fixed dollar amount, or something you adjust based on volatility? What made you choose that approach?