Few questions about the Dragonsoul system

[Mind Rapist 888]

It's been since last night I'm having those questions in my mind so I thought asking here. I've got a regular Dragonsoul system in the files but I've notices that the gem item (30270) gets dropped very easily from any mob you kill. So my question is where do I need to edit to make it harder to drop (20% chance let's say).

And about the mob level. Let's say the char is at level 70. How do I make it that this char can only drop the gem item from mobs of level 60 (char level - 10) or higher.