How about an alternative method using a Text Box to get what you want.
The first Text Box is drawn/inserted and sized at 14.85 x 22cm (wider width than the A4 size). Fine tuning the size can be done manually by inputting a size or by the up/down arrows and the Wrapping is set to Behind Text.
This size is half an A4 paper size.
If a user is using say Letter size in inches, they can change the dimensions to suit.

The Shape Outline/Fill colour and Weight can also be changed should a user wish to do so for clarity.
This half A4 size box, when selected can be positioned/moved using the arrow keys so ‘both’ sides and the top aren’t seen.

Text/content can be added to the top (half A4) to see how it looks with a chosen formatting and content won’t move the Text Box vertical dimension or position as more is added.

This created Text Box can also be resized, when selected, to 9.9cm x 22cm which is one third of an A4 page.
Once resized, it can be repositioned to the center of the A4 page by using the Position option in the Format group.
I have set the position to Position in Middle Center with Square text wrapping.
This Wrapping must be changed to Behind Text as before so any content added is over the Text Box.

This Text Box gives the illusion of two horizontal lines now which show the top, middle and bottom thirds which should give you what you want.
You could save the document (with inserted Text Box) as a Template for future use should you wish to.
Click the image below to see a short video of the above procedure.
Remember to hide/remove the Text Boxes before any printing or opt to temporarily un-check Print drawings created in Word in the Options. File > Options > Display > Printing Options > Uncheck ‘Print drawings created in Word’ > OK.