The numbers used in the calculations assume that all Bitcoin auctions sold at +3% above spot price. There has only been one auction winner that has come forward with the price they paid for their winning bid. Tim Draper claimed earlier this year "The market price then was $618 per bitcoin. At the last minute, I decided to bid higher than market price. I bid $632 and won all of the bitcoins!"
Obviously, every day there is (at the very least) a small amount variance in the Bitcoin price throughout the day, so the actual spot price during the auction would have been constantly changing. CoinMarketCap's all time chart was used to pull daily prices for each day an auction was on. Depending on what time of the day or how price is aggregated there will be a slight amount of variance, but given the large time horizon, and high amount of market variance daily, such variance seems negligible. The calculations could be made more accurate in the future by finding the median price on the day of the auction.