UPDATE: An interesting article by GATCOIN regarding the increase in ETHs value: https://www.gatcoin.io/hardcap-determination-announcement/
Here they cover the 7-day trailing average model they will be using, which I think is more than fair: "We will be using a 7-day trailing average, which means that we will look at 7 days of historical data for the price of ETH on coinmarketcap.com and btcaverage.com and then close, once we hit the USD market cap with that average." However, this also means there might not be an ICO at all, or if prices plummet it could go until closing date.