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Coverage percentage on its own does not tell you what closing the gap costs. Converting the shortfall into lines and then into developer days makes the trade-off explicit, and it usually reveals that the last ten percentage points cost more than the first fifty — because what remains is error handling, generated code and defensive branches that are hard to exercise.
Coverage gap
current = covered / total; lines to cover = ceil(total x target - covered); effort = lines to cover / lines per day
No. Coverage measures execution, not assertion quality. Chasing 100 percent produces tests that run code without checking behaviour, which adds maintenance cost and no safety.
Branch coverage is stricter, and mutation score is stricter still because it checks whether tests actually detect changed behaviour.