Just a quick update: it's about -1.985 now, while the all-time-low was -2.23. Still not in uncharted area. To break the ATL, the VWAP has to go below $173 today.
(This is based on bitfinex data as bitcoincharts.com has stopped reporting bitstamp data since the hack)
according to the OP formula calc, it went to -2.38 (new record) when BTC was $155 briefly. Right now -2.11 (at BTC=$203).
I will wait for the double dip and if we will not go below $155, it might be a time for a longterm buy.
Or not.