Hi Alan,
You can setup a flat rate on the Interest Settings on Sales Ledger module.
On "Calculation" field select "Flat rate per debiting" and add the rate in "Rate" field.
When you run the interest invoices maintenance routine it will create interest invoices with a fixed percentage, regardless of the days the invoice is overdue.
Please refer to this documentation for more details:
http://www.hansamanuals.com/main/manuals/mailnumber___3411/theconf___520/shortcode___HW0201SETTINGS_Interest/hwconvindex.htm
George Costa
PM HansaWorld