As you said, from curation rewards.
empo.voter will start a power down soon --> hive will go to 
From there it's fairly simple:
A huge % will be powered up.
The remaining % will be used to hold peg/add liquidity.
I don't have any concrete % because it all depends (if the price is already on peg or very close, most if not all Hive will be powered up), conservatively speaking I assume that the price of PWR will have to come down a bit once some delegators start selling. (0,85-0.9 would be a 'sweet' spot).
RE: PWR Update: Increasing the HIVE - PWR APR up to 15% (x3) and how to get the most from it.