To me, 20g seems like a lot for a 5 gal batch (28g=1oz) unless you have a real short contact time.
When I dry hop anything, I rarely use more than 1/4oz (that's 7g) for a 5 gal batch;
Around here, we call that "why didn't you dry hop it?".
I never use less than an oz., often 2-3 oz. of mixed varieties. And I frequently use my homegrown Cascades. I guess it just a question of different people. different tastes.

HA. I know what you're saying Denny...and you're right, as always it really just boils down to individual taste.
I know it seems low (and I guess in looking back, I
do use more than that in my own IPA)...but for most of the
regular quaffing ales I make, the 1/4 oz seems to do pretty well...I wind up with a nice floral aroma with no grassy-ness whatsoever. Depends a little on the varieties used, but the stated amount kept at cellar temps for a few days or a week does it for the average brew.
Even so (and again, it's just habit and personal preference) I tend to use a bit less dry hop than most, but will opt for longer contact time if need be. Lately I've been experimenting more with distilled hop oil for aroma as well...I'm getting pretty close to the result I'm really looking for with that.