  Forbush decreases (FDs) are short-term depressions in the galactic cosmic ray
flux and one of the common signatures of coronal mass ejections (CMEs) in the
heliosphere. They often show a two-step profile, the second one associated with
the CMEs magnetic structure (flux rope, FR), which can be described by the
recently developed model ForbMod. The aim of this study is to utilise ForbMod
to develop a best-fit procedure to be applied on FR-related FDs as a convenient
measurement tool. We develop a best-fit procedure that can be applied to a data
series from an arbitrary detector. Thus, the basic procedure facilitates
measurement estimation of the magnitude of the FR-related FD, with the
possibility of being adapted for the energy response of a specific detector for
a more advanced analysis. The non-linear fitting was performed by calculating
all possible ForbMod curves constrained within the FR borders to the designated
dataset and minimising the mean square error (MSE). In order to evaluate the
performance of the ForbMod best-fit procedure, we used synthetic measurements
produced by calculating the theoretical ForbMod curve for a specific example
CME and then applying various effects to the data to mimic the imperfection of
the real measurements. We also tested the ForbMod best-fit function on the real
data, measured by detector F of the SOHO-EPHIN instrument on a sample
containing 30 events, all of which have a distinct FD corresponding to the CMEs
magnetic structure. Overall, we find that the ForbMod best-fit procedure
performs similar to the traditional algorithm-based observational method, but
with slightly smaller values for the FD amplitude, as it is taking into account
the noise in the data. Furthermore, we find that the best-fit procedure has an
advantage compared to the traditional method as it can estimate the FD
amplitude even when there is a data gap at the onset of the FD.
