ok here is some calculations (from sport.be) I translated it and I hope it's clear!
Well it's pretty easy, in theory, because Justine 'only' has to win 2 or 3 matches to be sure she's in the semi's. If she gets there .. the number one spot is hers no mather what Kim does.
In every tournament players can win ROUND POINTS (the further you get the more points you get) and QUALITY POINTS (beating higher ranked players gives many bonuspoints). Before the start of the Masters Clijsters has 6596 points, Justine has 6461 (135 points less than Kim). Tha ranking only concludes the last 52 weeks so Kim and Justine will loose their points from last year. Kim won and got 485 round points + 265 quality points => 750 points to defend. Justine only got to the quaterfinal (121 round points) and got 35 quality points for beating dementieva so her total is only 156!!! The ranking shows this at the beginning of the Masters: Justine = 6426 points, Kim = 5967. Both girls get already 121 round points because they got the masterticket so Kim has to win 459 or more to stay at 1. If Kim wins the Masters she gains +/- 725 points (240 quality + 485 round points) it depends on the draw. If Justine gets to the semi's she will gain 266 points and that's enough to end the year as number 1.
Good luck Justine!
Roland Garros '03,'05,'06,'07
US Open '03,'07 / Australian Open '04
Olympic Gold '04 / YEC Madrid '06,'07