There are two methods, using a running total. First,if you reorder your detail fields (by changing the sort to the reverse of its current direction) so that the "last" record that you want summarized is your first, you can do the following:
Select the field you want to summarize {table.number}, sum, evaluate on change of group, reset never.
You could also define the particular record you want counted by using "evaluate using a formula" and entering:
{table.field} = maximum({table.field},{group.field}) //where {table.field} is a date or an ascending numerical field that defines the last record
Reset never. Both running totals belong in the report footer or a higher order group footer, depending on your report needs.
-LB