fluent adapting mesh for shock problem
 

I'm a Fluent beginner.
i have a problem in analysis on supersonic.
i will using adapting mesh in my problem and i studied fluent tutorial about adapting mesh.
in tutorial, they suggest me to select a gradient of static pressure, then in the normalization, select scale and 0.3 for coarsening and 0.7 for refinement.
But I wonder why it should be 0.3 and 0.7. Does Fluent just be set with those numbers for solving all kinds of coarsening and refinement for shock problem? Or is there any rule for finding numbers?