A good starting point is 1oz in 5 gallons of finished beer (for dry hopping). Once the beer is finished fermenting, and otherwise ready, add the dry hops to it. I also advise the hop bag for this, since it will contain the hops and make transfers easier.
I use 1oz of whole hops in a 3 gallon corny keg when dry hopping. I get what I want from that amount/ratio. I also add the hops to the keg right before the keg goes into the brew fridge to chill and carbonate (2 week slow carbonation method). This produces wonderful hop aroma/flavor in my glass for the entire keg. No need to remove the hops (at all) until you're cleaning out the kicked keg.
I'm not the only one that dry hops this way.