This method has not been authorised by anyone, it's just my own application on what would be a good idea for tipping competitions. There may be flaws in my reasoning. Feel free to email me with suggestions.
There are generally two ways to describing the result of a one day international: Team X wins by N runs, or Team X wins by N wickets. If a team wins by a number of runs, then this method does not apply. If a team wins by a number of wickets (ie the target is reached before the innings is complete) then the total of the team batting second should be extrapolated to determine the projected score if the innings was completed.
It should be noted that a team can win by a number of runs in a shortened match where D/L is used. In this case, it is possible that both teams' innings were shortened. Tipsters' margins are based on 50-over innings. In this application, if both innings are shortened to the same length, I have not worried about adjusting the margins to match the lengths of the innings, or extrapolated completed innings totals to 50 overs. However, where rain causes the second innings to be shorter than the first, D/L will be applied to compensate for the loss of overs, so that the innings totals are considered to be of the same length as the first innings. Where extrapolation of innings totals is required, the formula for the D/L Professional Edition is used, i.e., the current score is scaled up according to the amount of resources used, rather than using G50 in the calculation.
The tables provided freely by the ICC relate to the Standard Edition of D/L and only contain resource percentages for the beginning of each over. To calculate resource percentages where an innings is interrupted mid-over, linear interpolation of the given percentages is used.