Thing is that every situation differs. Sometimes the player might lightly touch the ball but hits more leg than ball. I tend to look at how far the ball actually travels from the point of the tackle. Don't know if it is the best way to determine, but I'm not always in the position to judge extactly how much ball and how much leg was played. If the ball barely moves, and the player goes down screaming, I call a foul.
Last week I had some guy trying to kick the ball away, in the process hits the opponent player very hard on his knee. I mean, he got the ball, a little. It wasn't a full on kick, however it was a full on kick on the knee.
Another example, one lady comes in very reckless from the front, gets the ball first, obviously, then takes the opponent down. I gave a free kick. Barely 10 minutes later, same situation, however this time the defender came in much much more careful and controlled, and thus I played on.
What I had another referee tell me once was that a nice response to: "I got the ball ref!!" would be: "onder andere". Which translates to: Among other things.