I often add a disc id for a release that is already in musicbrainz but has zero disc ids attached to it. Sometimes there are no track lengths or the track lengths match but often track lengths exist but do not match by a few seconds. Recently I have started using the “Set track lengths” link to alter the track lengths to that from the disc I have. I just want to make sure this is “correct” procedure. It seems reasonable to me that the timings taken off a disc are more authoritative than that gotten from some other source, but I can’t find anything in the documentation that supports this conclusion. Obviously when there are other timings based on other disc ids I don’t update the track timings based on my disc id.
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Yes it’s good.
If you are sure this is the exact same edition, add your disc ID and set track times.
If there is already a disc ID, though, you should create a new release for your specific edition, but it does not seem to be your case.
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That’s fine. It would be best if you also write the matrix code (what’s written on the matrix side of the disc) to the edit note.
This matrix code correlates with exactly one disc ID and identifies the disc from which track lengths were set, in case they are disputed (track lengths set a-new). 
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IMO this question also belongs to this topic.
What if MB says setting track times would alter the durations by milliseconds only?
Setting them anyway to show that it’s the matching discID?
Normally I forego setting them.
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I’d still set them. There are userscripts around that will let you see the milliseconds and can be useful to spot same releases\recordings
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