This is just what I have been told. The key to 
humidity is that whatever is comfortable to you, 
is likely OK for the cels as well. You don't want 
it to be too humid, and yet you don't want it to 
be too dry. If it gets too dry, the paint on the 
cels will stiffen and start to chip and crack. 
What I do is keep my cels in a setup like yours: 
art portfolio inside a zippered case and inside 
my closet. Other than that, I don't do anything. 
I don't know if the silica should be kept 
directly inside your case. I think having simply 
next to your case or just in your closet should 
be sufficient. It's a very good idea though and 
your cels look great!