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A 500-MHz whole-body magnetic-resonance-imaging magnet has been under study for the Iseult/Inumac project since 2006. The central field of this magnet must be 11.7 T in a warm bore of 900 mm. The required field homogeneity is 0.5 ppm peak to peak in a sphere of diameter 22 cm. An innovative winding method using double pancakes (DPs) was devised to handle these outstanding specifications. The choice of the used method will be explained. Among the different sources that can generate field errors, we shall focus on the winding errors while manufacturing. The accuracy on the magnetic field must be less than a few parts per million, which implies precisely evaluating the effects of these errors on the field quality. Therefore, different methods were developed in that aim. The principle of each of these methods will be given. A code that links the winding data to the errors on the spherical harmonics expansion was then written. To check the availability of the results, the code was applied to several cases whose exact solution is known. Finally, the code was applied to wound DPs for the prototype H0 of Iseult. A first estimation of the mechanical tolerances on the winding will be done.