trader101 uses beginning of the week as a basis of comparison. He measures how does each pair changes with reference to the beginning price of the week.
Each pair will performs positively or negatively but so long if 7 long pairs and 7 short pairs are in perfect natural hedge, we shall see losses in term of spread.
The most important element of T101 Basket Trading System is the relative gain and loss of each pairs. The absolute gain/loss gives the idea of whether the market is trading in range or trending strongly. Not long after the thread started, trader101 suggested to use 100pips for the anchor pairs for this purposes.