  Gravitational microlensing has the potential to provide direct gravitational
masses of single, free-floating brown dwarfs, independent of evolutionary and
atmospheric models. The proper motions and parallaxes of nearby brown dwarfs
can be used to predict close future alignments with distant background stars
that cause a microlensing event. Targeted astrometric follow up of the
predicted microlensing events permits the brown dwarf's mass to be measured.
Predicted microlensing events are typically found via searching for a peak
threshold signal using an estimate of the lens mass. We develop a novel method
that finds predicted events that instead will lead to a target lens mass
precision. The main advantage of our method is that it does not require a lens
mass estimate. We use this method to search for predicted astrometric
microlensing events occurring between 2014 - 2032 using a catalog of 1225 low
mass star and brown dwarf lenses in the Solar Neighborhood of spectral type M6
or later and a background source catalog from DECaLS Data Release 9. The
background source catalog extends to $g = $ 23.95, providing a more dense
catalog compared to Gaia. Our search did not reveal any upcoming microlensing
events. We estimate the rate of astrometric microlensing event for brown dwarfs
in the Legacy Survey and find it to be low $\sim10^{-5}$yr$^{-1}$. We recommend
carrying out targeted searches for brown dwarfs in front of the Galactic Bulge
and Plane to find astrometric microlensing events that will allow the masses of
single, free-floating brown dwarfs to be measured.

Solar and Stellar Astrophysics;Earth and Planetary Astrophysics;Astrophysics of Galaxies