I'm not familiar with Zombie Dust in particular, but Citra and Cascade are really different hops.That being said, you can't go wrong with as much Cascade hops as you can pack into a recipe.
Personally I'm not a big fan of Citra. I like the aroma, but I get a kind of artificial sweetner note from the flavor. I'm going to start subbing Mosaic for Citra in the recipes where I've used it.
All Cascade is going to give you something more like Sierra Nevada Pale Ale. I did a Cascade SMASH not too long ago that I consider one of my best and favorite beers.
PS...Cascade and Citra together is quite a good combination, IMO and if you use a little Citra somewhere in the late addition along with a bunch of Cascade, it might play up some of the more subtle flavors in the Cascade.