Consider 3 futures contracts:
A BTC/ETH, settled in BTC B USD/ETH, settled in ETH C USD/BTC, settled in BTC
As the markets aren't efficient, sometimes these contracts might move apart from each other.
To calculate the spread, I believe the formula would be
spread % = A / (B/C - 1) * 100%
If the spread is positive, and you want to earn the spread, which contracts should be long & short?
(I believe it should be long A, short B, long C)