Im having trouble with a running sum for my VAT analysis section of a report.

Whats happening is the vat is calculated and rounded appropriately. But the running sum seems to be calculated on the pre rounded figure, is there anyway to rectify this.

So i have following figures*245.70 at 21% which is 51.597 when rounded goes to 51.60
120 at 21.5% which is 25.80 with no need to round
382.20 at 13.5 which is 51.597 when rounded goes to 51.60*

i want it to add the rounded figures so so 51.60 + 25.80 + 51.60 =129.00. So it looks OK on the report

Instead it is displaying 128.99 which is rounded from 128.994 since (51.597 + 25.80 + 51.597) = 128.994

Any help on the issue is much appreciated.