Instead of using an inserted box, insert a text box. Do not stretch it in height. Place it in the report footer and stretch it to fit the report width only. Right click on the text box->format text->check "can grow" and uncheck "keep object together". On the borders tab, set the borders to single line. Then create these formulas:
whileprintingrecords;
numbervar cnt := cnt + 1;
Place this formula in the detail section. If there any group sections that appear on the page, you should also place a copy of the formula in these. Then create a formula {@lines} like this:
whileprintingrecords;
numbervar cnt;
if onlastrecord then
replicatestring("______________________________________________________________________________________________________" +chr(13),60-cnt);
Place this formula in the text box in the report footer, in the left hand corner.
In the section expert, format the report footer by unchecking "keep together."
Note that you need to adjust the 60 to whatever number would coincide with the number of rows on the full page if it were filled with data--play around with this until you get the right match, with no lines extending onto a second page.
-LB