A group grading on a matrix algebra

is called
good if all the matrix units

are homogeneous
elements. We present a new way to classify good

-gradings by the orbits of a certain action of the
group

on a set of

-tuples of non-negative
integers, and we use it to count the isomorphism types
of good

-gradings on

in the case where

is a cyclic group of prime index

.