This was ratified at the July 1995 ICC meeting:

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10.3 Delayed or Interrupted Matches - Calculation of the Target Score

If the innings of the team batting second is delayed or interrupted and it is not able to receive its full quota of overs, the target score shall be calculated as follows: the score of the team batting first shall be multiplied by the pecentage factor for the number of overs to be bowled to the team batting second, as set out on the Target Score Calculation Chart (Appendix A). Fractions shall be rounded to the next higher whole number.

The percentage factors have been derived from a detailed from a detailed mathematical analysis of a database of one day matches with the object of establishing "normal" performance.

APPENDIX A

TARGET SCORE CALCULATION CHART

Overs           % Factor
25              66.7
26              68.4
27              70.2
28              72.4
29              74.2
30              76.0
31              77.8
32              79.1
33              80.9
34              82.2
35              84.0
36              85.3
37              86.7
38              88.0
39              89.3
40              90.7
41              92.0
42              92.9
43              94.2
44              95.1
45              96.0
46              96.7
47              97.8
48              98.7
49              99.6
50              100.00

e.g. Team batting first score 188. The innings of the team batting second is limited to 37 overs:

188 * 86.7% = 162.996

Target Score = 163 runs

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Note: This gives absolutely no indication of how a match should be treated when the first side does not face 50 overs, and the second side faces even fewer overs. For example, in the 1995/96 India-Pakistan match in Singapore:

India: 226 off 47.1 overs Pakistan Target: 187 off 33 overs

Here is how I made it work:

Target = 226 * ( 33 over factor / 47.1 over factor )
= 226 * ( 80.9 / 97.9 ) where 97.9 is an estimation of the 47.1 factor (47 ov = 97.8%)
= 186.75587
= 187

ODIs in Australia also do not use the above rain rule. The Australian rain rule that is still in use is to take the highest scoring overs of the team batting first, and then to deduct the target by 0.5% per over lost.