The rising interest in free-piston engines and compact electrical generators, as they are used for range-extenders in the automotive industry, advances the development of high-frequency high-thrust tubular linear synchronous generators. Previous studies suggested a Vernier machine with Quasi-Halbach-magnetization as the most promising topology. Its analytical design discloses new challenges, as small pole pitches paired with large air gaps render various conventional approaches ineffective. This article compares two methodologies for the calculation of the air gap field combined with three permeance functions, considering the slot geometry of the stator. It is shown that the accuracy of the results highly depends on the underlying geometry and topology. With the given design recommendations, the thrust force can be calculated with an error of maximum 9.3 % compared to FEA results.