Antonyms for the word "guesstimating" include "calculating", "measuring", "evaluating", "estimating", "quantifying", "determining", and "assessing". Unlike "guesstimating", these words imply a more accurate and informed approach to determining a value or making a prediction. "Calculating" involves using mathematical operations to determine a precise answer, while "measuring" involves using instruments to determine a specific quantity. "Evaluating" and "assessing" imply a thorough analysis of all relevant factors before arriving at a conclusion, while "quantifying" and "determining" suggest a process of precise measurement or calculation. These words, therefore, provide a more reliable alternative to the often approximate and uncertain process of "guesstimating".