According to US GAAP and IFRS guidelines, the most appropriate valuation methodology is a price that is highly correlated with an executable price in contrast to an average price (including a VWAP, TWAP, etc) which is not considered to be fair market value.   Additionally, ASC 820 (FASB, 2011) indicates that if there is a principal market for the asset or liability, the fair value measurement shall represent the price in that market. For a benchmark to be relevant, the benchmark price should be at least approximately tradable on an accessible market