[en] Variable-gap undulator systems are widely used in storage rings and linear accelerators to generate soft- and hard X-ray radiation for the photon science community. By closing the undulator its focussing effect gains in strength, which needs to be corrected, in order for the optical functions in downstream parts of the accelerator to be constant. At the free-electron laser (FEL) facility FLASH at DESY two undulator systems share one common electron beamline. The first undulator is a variable-gap system used for seeding experiments, the second undulator is a fixed-gap system, which serves the user facility with FEL radiation. Variation of the gap in the first undulator results in altered beam optics, which deteriorates the FEL process in the second undulator. This gives rise to the need for a method to calculate a steady correction function for quadrupole currents depending on the current gap size, effectively making the disturbance transparent for subsequent sections. The approach presented here applies the implicit function theorem to an analytical model of the beamline section and can be used for any pertubation of the beam optics. In this contribution we present the method and its implementation as well as measurements performed at FLASH.