I googled that and I checked my FX's gain in this year. Value f was 0.08. By some chance, my curriculation is wrong, but the result said "you bet 23% of your money." I using "2% rule", so I was very surprised. If I lose 4 times, I lose almost all money for FX.
I guess the web site which I searched was wrong. So I need the true information from book or something.