  Twisted structures of chiral cubic ferromagnetics MnSi and Cu$_2$OSeO$_3$ can
be described both in the frame of the phenomenological Ginzburg-Landau theory
and using the microscopical Heisenberg formalism with a chirality brought in by
the Dzyaloshinskii-Moriya (DM) interaction. Recent progress in quantum
first-principal methods allows to calculate interatomic bond parameters of the
Heisenberg model, namely, isotropic exchange constants $J_{ij}$ and DM vectors
$\mathbf{D}_{ij}$, which can be used for simulations of observed magnetic
textures and comparison of their calculated characteristics, such as magnetic
helix sense and pitch, with the experimental data. In the present work, it is
found that unaveraged microscopical details of the spin structures (the local
canting) have a strong impact on the global twist and can notably change the
helix propagation number. Coefficients ${\cal J}$ and ${\cal D}$ of the
phenomenological theory and helix propagation number $k={\cal D}/2{\cal J}$ are
derived from interatomic parameters $J_{ij}$ and $\mathbf{D}_{ij}$ of
individual bonds for MnSi and Cu$_2$OSeO$_3$ crystals and similar cubic
magnetics with almost collinear spins.
