2.1 shares of x for every share of rock if you can catch the market when the prices are lined up that way.
just divide the price of rock by the price of x, and if you get anything close to 2.1, enter at the market.
as an example, you would short 100 shares of rock and buy 210 shares of x
but obviously you can exit at any time. From the close on the 22nd to the close on the 23rd. the trade is up almost $200 but that's mostly because both sides are showing profit
and is not something that happens often.
I would also exit if the ratio exceeds 2.25 at the close of any day. That would result in a loss of about $350. SO the risk reward using those parameters is very close to 2 to 1