ars est recta ratio factibilium (regarding making)
prudentia est recta ratio agibilium (regarding action)
scientia est recta ratio scibilium seu speculabilium (regarding knowledge)
The key here being recta ratio which needs also to be defined.
recta here means right or proper, good in the sense of proper proportion and right relation
ratio here means plan, pattern, mode of action that has been reasoned out, exemplar idea
And the overarching principle here is that ars, prudentia and scientia are virtues: i.e. habits of mind and soul (i.e. dispositions) toward right action.
Cf. Roy J. Deferrari, A Latin-English Dictionary of St. Thomas Aquinas: Based on The Summa Theologica and selected passages of his other writings, St. Paul Editions: Boston, 1986, 893. 889.