How to make a power get better (other than damage) depending on the level of weapon equipped?

How to make a power get better (other than damage) depending on the level of weapon equipped?

I am making my own campaign and I want to add back the power from alpha_demo to raise your shield. I did this without trouble, however the power is incredibly powerful against low-level enemies and terrible against high-level enemies. I want the reflect chance and absorb increases to depend on the level of the shield. How do I do this?