🚀 go-pugleaf

Hello Dominic,
The original poster prepared a document in Frame (only he knows what the paragraph formatting setup was). Afterwards the poster found out he needed line numbering. There started the difficulty.

If the original poster just took a Frame template, as shipped with the product, he'll have irregular line spacing. Just check out the default portrait template in Frame:
Body text is 12pt on 14
Title is 18pt on 24 with 24 above and 12 below
Heading 1 is 14pt on 16 with 14 above and 6 below.

Immediately you can see that the position of a line of body text on a given page now depends on how many Titles and Headings are on the page. It certainly will not be regular 14pt spacing except on pages with body text only.

I've just looked at quite a few templates shipped with Frame and could not find one that generated regular line spacing. That is body text always falls regularly regardless of other para formats on the page.

It's clear that if the original poster knew he needed line numbering then all the paragraph formatting could have been setup to provide regular multiples of the body text line spacing. That way, either a master page solution or Riley vanDyke's solution would have helped the original poster.

My suggestion about having one body text line per paragraph was a measure that would work if the original poster has a body text format of say 12pt on 14 with a 6 point inter para space. This is not a rare format hence my suggestion, which I did say would be a real nuisance, but it avoided explaining all the typography issues around the original poster's predicament.
FWIW
Ian