Hi Quasar,
This is what I do to create a Numbers Layout Guide for a one-page presentation.
Numbers > Preferences > Rulers

Tick all boxes and leave them ticked. You will be glad that you did!
Numbers Menu > View > Show Rulers
If you like, drag Alignment Guides (the yellow lines) to the right from the vertical ruler, and downwards from the horizontal ruler.
They mark the left and the top margins.
Calculate (or use trial and error) to mark the right and bottom margins.
Insert a rectangle Shape (no fill) and drag to enlarge it to where you have calculated (or guessed) the page margins.
Test with Menu > File > Print... and adjust.
When all is satisfactory, Save as Template for future use with other documents that have the same paper size and margins..
Here is an example


Click on the "Done" button to escape the non-editable Print Setup. Delete the rectangle Shape (temporary page layout guide) before printing to paper or PDF.
Did you remember to Save as Template after all this hard work?
Regards,
Ian.