I understand the basics of pair trading. However, there's something I can't resolve and I haven't seen it resolved in anything I read so far online. Say I want to make a pair trade with FDX and UPS. Now, I get their historical prices for one year, find the ratio of the two prices, the mean, and the standard deviation. Now, say that right now the ratio is two standard deviations away from the mean so I make the trade. The objective is to close the position when the ratio returns back to the mean, but here comes the problem. I can check every day for whether the ratio reached the mean or not but which mean do I check? The mean that was used when the position was originally opened or the mean of all the ratios since a year before the position was opened until now?