The phrase "makes up off the top of head" refers to the act of improvising or coming up with something quickly and without much thought or preparation. Antonyms for this phrase would be terms like "planned," "calculated," or "deliberate." Other related antonyms might include words like "thorough," "analytical," or "meticulous," which indicate a careful and thoughtful approach to problem-solving. Additionally, antonyms like "predictable," "formulaic," or "rote" could be used to describe situations where improvisation and spontaneity are not valued or are considered undesirable. In other words, antonyms for "makes up off the top of head" would portray a more structured, well-planned approach to decision-making and creative problem-solving.