Crystal: summarising formula fields

For some crazy reason, it seems that Crystal won't easily let you summarise (sum, average, etc) calculated values. After trawling the net for a while I managed to come up with the following 'hack' to achieve this.

1. Create a calculated field that clears an array variable & returns "";
2. Insert this field into a group or report header;
3. Create a calculated field that re-dimensions the array variable with an extra element, adds the calculated value to the array & returns "";
4. Insert this field into a details line;
5. Create a calculated field that summarises the array variable & return this out put;
6. Insert this field into a group or report header.