To evaluate the bias and precision of the respiratory muscle training device formulas to predict respiratory minute volume (RMV) and volume of the reservoir bag (BV) on a cohort of subjects with Cystic Fibrosis (CF). CF patients with available pulmonary function tests and maximal voluntary manoeuvres were included in the study. Vital capacity and maximal voluntary ventilation were extracted from subjects’ records and then inserted to the manufacturer’s formulas to obtain RMV and BV (measured setting). RMV and BV were compared according to standard and measured formulas in males and females. Sample was described and then processed using Bland–Altman analysis. Bland–Altman analysis for RMV revealed a bias and precision of 8.8 ± 29 L/min in males and 28.8 ± 16 L/min in females; 0.4 ± 0.5 L in males and 0.7 ± 0.4 L in females for BV. Concordance correlation coefficients for RMV were −0.03 in males and 0.02 in females; 0.22 in males and 0.03 in females for BV, reinforcing an unsatisfactory concordance between measured and manufacturer setting. This study shows considerable discrepancies between the two methods, making the degree of agreement not clinically acceptable. This might cause inappropriate setting and disservice to patients with CF.