Overview Map
The exact size (scale) of the overview map in the frame cannot currently be calculated, an estimated size (scale) is used.
This intermediate solution (scale calculation) will be omitted with version 1.1 (included as of WebOffice 10.9) of the VertiGIS Studio Printing Engine tools.
The red rectangle in the overview map which indicates the print area is not calculated by WebOffice, but it can be defined in the layout as an extent indicator.

Using an Extend Indicator on the layout in ArcGIS Pro - to display the print area in the overview map on the printout.
The following section shows how to add the extent indicator to the print template. By means of the Extent Indicator the displayed map section is also shown in the overview map.
If the Extent Indicator field is not yet available on the template, another adjustment must be made to the template.
1.First, a new map is created via Insert (New Map).

Insert a new map in ArcGIS Pro
2.In Catalog (in the Maps pane) you can check if the new map has been added successfully.

View in the ArcGIS Pro Catalog
3.Then select the WEBMAP_MAP_FRAME element (in the Contents pane) and select the new map from the MAP_FRAME drop-down menu:

Adjustments to WEBMAP_MAP_FRAME in ArcGIS Pro
4.Afterwards the element OVERVIEW must be selected. Now the function Extent Indicators is enabled. The element WEBMAP_MAP_FRAME is selected.

Selection WEBMAP_MAP_FRAME via Extent Indicators in ArcGIS Pro
As can be seen from the figure, the OVERVIEW is supplemented by the Extent Indicator. If you want to display the same map service in the overview map, you have to select the Map element (under Extent of WEBMAP_MAP_FRAME) and delete the map services listed there.
5.In the last step, the WEBMAP_MAP_FRAME element is selected and the map frame is reset to None.

Reset the map frame to <None>
In order to be able to save the adjustment permanently, the print template must be saved again as a new layout file under ..\arcgisserver\directories\VertiGIS_PrintLayouts with right click - Save as Layout File.