Stock pack height?

Depends on the stock pack. Some more than others and some have the ability to add more height if needed. As a basic number maybe 1/2". The Triad stock pack you can add strips under it to raise it more.

 
I've been using the cheap Vudoo tactical ones for my kids. They come with spacers to add height.


You can always just tape some foam under it to build it up. Doesn't have to be pretty if you're covering it right?