A note saying “line 18” seems precise until you open another edition and find a different sentence there. The number may describe a particular arrangement of the text rather than a reference shared by every version.
Identify the numbering system
Check whether the source counts manuscript lines, printed lines, verses or editorial sections. A reflowed webpage can display the same passage without preserving the visual line breaks of a printed edition. Do not substitute one system for another without explanation.
Add a short locating phrase
Alongside the edition, page and numbered reference, record a few opening words for your own retrieval. Keep quotations brief and accurate. The phrase is a finding aid, not a replacement for a full reference when you publish an argument.
Test the note by closing the source and locating the passage again. If you cannot return to it reliably, improve the reference now. Historical reading often involves several translations and editions; a small investment in precise locations prevents later comparisons from accidentally discussing neighbouring but different passages.