Purpose: This paper approaches the calibration procedure for EBT3 radiochromic films, and its precision, using a flat-bed scanner as a digitization system, for routine QA in a Superficial Therapy X-Ray machine. Methods: This paper analyzes precision in EBT3 radiochromic film calibration, using a flat-bed scanner as a digitization system, and describing routine control procedure within the framework of a quality assurance program for X-Ray equipment. Radiochromic films were cut into 5cm x 5cm squares, placed on 5cm of PMMA at 15cm source-film distance, and irradiated with several doses to obtain the calibration curve of net optical density (ODn) as a function of dose, and dose uncertainties were computed by residual analysis. Results: The calibration curve fits a fourth-degree polynomial function, with uncertainties of 2.10%, and 2.48% for the dispersion in the dose measurements in the films; we also considered a 3.5% uncertainty in dose measurements due to the use of an ionization chamber, for a net uncertainty (one sigma) of 4.77 % in the dose value. Conclusions: The calibration curve of the EBT3 films was obtained by a procedure that establishes the ODn as a function of the dose delivered by the X-Ray machine, based on the traceability of the dose.