I would create a formula with a shared variable to carry the amounts, DBlank would probably offer up running totals.
Formulas like this usually come in groups of 3, but yours might be only 2.
Usually there is a reset somewhere (usually a group header):
shared numbervar CF:=0;
""//to hide the zero on the report.
An incrementor (usually in details)
shared numbevar CF;
CF := CF + {table.field};
""//though if you want to see this number remove the ""
A reporter (usually in footer)
shared numbervar CF
HTH
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