Basic idea for the formula: USD buying power of Bitcoin vs altcoin (in this case, Nano). When the chart is bullish, best to hold USD or BTC. When it's bearish, best to hold Nano.
USDTBTC/NANO just hit 1.618 fib level. Would bet there is some resistance here, and the formula heads back down to the 1.0 mark (or, in other words, good news for Nano holders).