I like a pair trade long x and short rock with the same dollar amount on both sides. A good entry would be
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