We made software to simulate Larmor precession in a setup for SESANS with adiabatic/RF flippers in magnets, existing at PNPI. The final polarisation of a divergent “ribbon beam” of height 2 cm is calculated as a function of λ. For λ= 6 A, flippers 56 cm apart and RF frequency 1 MHz we find spin-echo length δ= 0.9 μm. We show numerically, how λ is converted to δ. Extension to δ= 20 μm is realistic.