Hi Meiyappan,
When you say that exchange rate is not always correct in BW system does it mean that you are not importing exhcnage rates from SAP in BW frequently?
I think you should follow SAP note and for exchange rate you can extract them daily by scheduling the program RSIMPCURR in BW system that will refresh the exchange rates daily and your calculations will not be impacted if you do them in transformation.
Thanks
Amit