wall/colloid cutoff value question

In fix wall/colloid, is the cutoff distance is from the surface of the wall to the surface of the colloid or the center of the colloid? In the example (fix zwalls all wall/colloid zlo 0.0 1.0 1.0 0.858 zhi 40.0 1.0 1.0 0.858), the cutoff value of 0.858 make senses if the radius of the colloidal particle is 0.5 and sigma is 1. As far as I understand, there will be no interaction when the surface of the particle is more than 0.358 units apart. I am using a colloid with diameter 20 sigma. What is an appropriate cutoff value?

From the doc page for fix wall/colloid:

For the wall/colloid style, epsilon is effectively a Hamaker constant
with energy units for the colloid-wall interaction, R is the radius of
the colloid particle, D is the distance from the surface of the
colloid particle to the wall (r-R), and sigma is the size of a
constituent LJ particle inside the colloid particle. Note that the
cutoff distance Rc in this case is the distance from the colloid
particle center to the wall.