- # calculate tax and rounding error for the whole group
- my $extra_cents = sprintf('%.2f', $taxable_cents * $tax_object->tax / 100)
- * 100 - $tax_cents;
- # make sure we have an integer
- $extra_cents = sprintf('%.0f', $extra_cents);
+ # calculate tax and rounding error for the whole group: total taxable
+ # amount times tax rate (as cents per dollar), minus the tax already
+ # charged
+ # and force 0.5 to round up
+ my $extra_cents = sprintf('%.0f',
+ ($taxable_total * $tax_object->tax) - $tax_cents + 0.00000001
+ );