Yesterday (Sept 14th, 2006) India lost against West Indies in the match conducted at Malaysia.
There is not allowance given in terms of field restrictions, where typically 20% of runs are made extra in the Power-plays. So in effect West Indies, had made only 113 (-20% on 141).
Of course there is an issue of team batting second, with a reduced score, where the pro-rata approximation can be used.
I would re-device the method like this.
Say if the team 1 batting first bats the full 50 overs, in the alloted time.
(1) If there is rain interruption after the team 2, bats for greater than 20 overs for a result. Take out 20% of runs, or the difference in run rate of the opposition.
India bats first and makes 309/5 and 133/2 in 20 overs, so the run rate difference is 6.65 - 6.18 = 0.47 or 4.7% less. Hence the actual run made by West Indies is 141/2 ( - 20% + 4.7% ) = 119. So in effect India won, by 14 runs.
(2) If there is rain interruption before the second session, and there is loss of say 30 overs, due to time. So the team batting second has to make 20% more than the average run-rate needed. For example, India bats first and makes 309/5 and 133/2 in 20 overs, the target has to be 6.2 + 20% = 7.64, which means West Indies should be given a target of 153. Even with this, WI made only 141, hence lost by 12 runs.
Note - the nearest whole number should be used.