I do not know of a good way to do this in general (locating the position on the print page of the last row).
But if what you want to do is to draw a line on the border of the bottom row, you can do that from within the grid using PrepareViewStyleInfo.
private void gridControl1_PrepareViewStyleInfo(object sender, GridPrepareViewStyleInfoEventArgs e)
{
if(this.gridControl1.PrintingMode && e.RowIndex == this.gridControl1.RowCount)
{
e.Style.Borders.Bottom = new GridBorder(GridBorderStyle.Solid, Color.Red, GridBorderWeight.Thick);
}
}