Your own number has no scale
Being named in 24% of answers is neither good nor bad on its own. It is good if the leader sits at 30% and bad if the leader sits at 80%. Without competitor figures from the same runs, a visibility number is a reading with no units.
Count both sides identically
The most common error is asymmetric counting: your brand gets credit for any appearance, while competitors are only counted when recommended. That single asymmetry can double an apparent share. Use one rule for everyone - typically "named in the answer body" - and apply it without exception.
The same applies to the question set. Share calculated over your questions, where competitors are counted over a subset, is not a share.
A mention is not an endorsement
"Avoid Brand X, it has poor support" is a mention. Counted naively it improves your visibility figure while describing a commercial disaster. Sentiment has to be recorded alongside the mention, and negative mentions reported separately rather than netted off.
Who counts as a competitor
Only brands a buyer could realistically choose instead of you, in the same category. Marketplaces, directories, media outlets and review sites are not competitors even when they appear constantly - including them makes your share look small for no useful reason. Equally, omitting a real competitor silently inflates your share.
Expect the list to be wrong at first
The first round usually surfaces two or three brands you did not consider competitors but models clearly do. Add them to the list, note the version change, and treat the model's framing as information about how the market is perceived.