I have to use a Planned % Completion in programmes, with the (fairly) standard formula

Int(IIf([Status Date]>[Baseline Finish],100,IIf([Status Date]<[Baseline Start],0,ProjDateDiff([Baseline Start],[Status Date])/ProjDurValue([Baseline Duration])*100))) & "%"

This works fine for individual tasks, but does not give a %age for summaries. The exception to this is any works added after the baseline was set, which do give a percentage (of 0%, obviously).

I know there are other methods od calculating, but this is info that is filled in on a spreadsheet that then leads to graphs, pie charts etc to a standard format that is client required, so I cannot use EVM or any other alternative- it has to be planned % complete based on th eoriginla baseline.

Any help would be appreciated, and to clarify my worjk computer is tuning MSP10 standard.
The method of evaluation is client driven and has been mostly adopted by the company, so alternatives are not really an option.