The same summary field can be used to compute a variety of sub-totals and also a grand total at the beginning or end of your report as long as the records being summarized are part of your current found set.
Here's a link to a simple tutorial on setting up summary reports that you may find useful:
Creating Filemaker Pro summary reports--Tutorial
Feel free to post back with more questions if this doesn't get you all the way there.
Thanks for the reply and the link to the tutorial. it certainly makes more sense now and I have successfully created my report without the creation of lots of new fields. The tutorial is a lot easier to follow than the 'Help' screens, and perhaps the biggest revelation was the fact that you can delete the 'body' of the report and just leave the sub-summaries.