Possibility problem

So I am making this mod where you have a gold pan and when you right click it on water (flowing) it has a chance to drop a gold sprinkle (2 gold sprinkles make 1 gold nugget) but it has a better chance of dropping useless river dirt... The problem is, I configured a procedure stating when rightclicked on flowing water it only spawns but when I right click on anything really it drops dirt. Thats weird because in the configure screen I set the 'blocks affected option' to only flowing water. Can anyone help? Also how do I decrease/increase the chance? Is  more and lower than 0.5 really easy and more and lower than 19.7 really hard? Thanks.