I went ahead and added an additional 2.5AAUs of Cascade at knockout. Original recipe had 2.5 at 30 mins and 2.5 at 15 mins. Haven't decided on dry hopping yet but I don't intend using a secondary which seems to be recommended for dry hopping.
Stuff I learnt in my second batch:
How big do those little hops get when they fill with water!
I need to find a better way to get an OG sample. After aeration, the wort had a good 7-8 inches of foam on top. I had nothing to take a sample that didn't involve me putting my hands in the foam so I decided not to take a sample as it was the lesser of two evils.
Also because of the foam I'm really glad I rehydrated the yeast. I think the powder would have just got suspended in the foam if I had sprinkled it straight on to the wort.
24 hours later it's bubbling away with a pleasant hoppy aroma coming from the airlock.