Brewed 10 bbl's of IPA @ 17.3P. Pitched US-05 harvested from a previous batch. When I harvested I noticed that the harvest was unusually runny but had no fresh yeast on hand at pitching so pitched anyway. Ferment started as normal 70 deg then down to 68. At day 3 noticed ferment slowing and capped tank on day 4. It's day 5 now, took a reading and beer was up at 10.8P! Usually we're at or about terminal gravity after 5 days. Immediately released pressure from tank (had gotten up to 20 psi which would be normal for one day for a beer capped at 5.5 - we usually attenuate down to 3.5 - but not for a beer at 10.8).
It seems fairly obvious that I underpitched so the question is what to do now. I can pitch some more US05 from a fresh brick but my concern is the CO2 in the tank. Do I aerate the fresh yeast first or just pitch dry into the fermenter? Also, do I pitch the whole .5 kg pack or just a portion and if so, how much?
I should also say that I'm under the gun to get the beer out so waiting another week to see what happens is a very unhappy option.