  The spin-orbit interaction strength g_so in helical magnets determines both
the pitch wave number q and the critical field H_c1 where the helix aligns with
an external magnetic field. Within a standard Landau-Ginzburg-Wilson (LGW)
theory, a determination of g_so in MnSi and FeGe from these two observables
yields values that differ by a factor of 20. This discrepancy is remedied by
considering the fermionic theory underlying the LGW theory, and in particular
the effects of screening on the effective electron-electron interaction that
results from an exchange of helical fluctuations.
