A line score is good when it meets or exceeds the threshold for the job you want. There is no single target, because thresholds are set per MOS and per line score. The examples below are drawn from the 119 verified Army MOS on this site and are the actual lowest, median, and highest recorded requirements for two of the most used line scores.
ST (Skilled Technical), used by 46 jobs: the lowest requirement is 94P Multiple Launch Rocket System Repairer (EL ≥ 93 or (EL ≥ 88 and ST ≥ 85)), the median is 74D CBRN Specialist (ST ≥ 100), and the highest is 35N Signals Intelligence Analyst (ST ≥ 112).
GT (General Technical), used by 30 jobs: the lowest requirement is 89B Ammunition Specialist (ST ≥ 91 or (ST ≥ 86 and GT ≥ 85)), the median is 14H Air Defense Enhanced Early Warning Operator (MM ≥ 91 and GT ≥ 90), and the highest is 31D Criminal Investigation Special Agent (GT ≥ 110). 3 of the 30 GT jobs ask for 110 or more, which is why a GT of 110 is often quoted as the line between "most jobs" and "the technical and intelligence fields".
Across all 119 jobs, the most used line score is ST (Skilled Technical), which appears in 46 rules with thresholds from 85 to 112. The Army score guide explains what each line score measures and lists the lowest, typical and highest requirement for every one; the full picture by score range is on the jobs by score page; each MOS page shows its own rule, source and check date.