Two different approaches are discussed for computing the expected ELMER image quality. The final Error Budget is computed in two different ways: a hierarchical model and a sequential model. The hierarchical model is the classical process for error budget building, in which the requirement degradation due to each error source is analyzed independently, combining all errors at the end. The sequential model follows the real design and fabrication process, simulating the steps sequentially. The results obtained in each step feed the following one, degrading the requirements from the Encircled Energy Radius (EER) design value to the final one. Finally, both results are compared.