Based on the jLT wiki, you can learn toolbox mastery (master) at lvl 60/62/64/... and the maximum level for the skill is 50. In addition, it costs 2 jps for each level. According to the comments at the bottom, the skills add a fixed amount, not %.
Link: Skill/エンジニア - ラテール Wiki*
With my limited Japanese knowledge, the last comment of the skill roughly translated to: "You can get over 470 max. damage with a Harpie bag+9"