  To identify under what conditions guiding-centre or full-orbit tracing should
be used, an estimation of the spatial variation of the magnetic field is
proposed, not only taking into account gradient and curvature terms but also
parallel currents and the local shearing of field-lines. The criterion is
derived for general three-dimensional magnetic equilibria including stellarator
plasmas. Details are provided on how to implement it in cylindrical
coordinates, and in flux coordinates that rely on the geometric toroidal angle.
A means of switching between guiding-centre and full-orbit equations at first
order in Larmor radius with minimal discrepancy is shown. Techniques are
applied to a MAST (Mega Amp Spherical Tokamak) helical core equilibrium in
which the inner kinked flux-surfaces are tightly compressed against the outer
axisymmetric mantle and where the parallel current peaks at the nearly rational
surface. This is put in relation with the simpler situation $\vec{B}(x,y,z) =
B_0 [\sin(kx) \vec{e_y} + \cos(kx)\vec{e_z}]$, for which full orbits and lowest
order drifts are obtained analytically. In the kinked equilibrium, the full
orbits of NBI fast ions are solved numerically and shown to follow helical
drift surfaces. This result partially explains the off-axis redistribution of
NBI fast particles in the presence of MAST Long-Lived Modes (LLM).
