First calculate the expectancy of your system. That should give you an indication of what you can risk per trade. (Win % x Average Win Size) – (Loss % x Average Loss Size)
5% -15% is insanely high. Besides making profits, capital preservation is also very important. Here’s a rough calculation, if you risk 2% per trade, it will take 50 losing trades to wipe your account. However by risking 5 or 6% you will blow your account just after 4 losses. I never risk more than 3% per trade but many traders I know prefer to follow the 1% rule. I think it’s good to keep the value somewhere between 1 to 2% and adjust it before every trade.