During the production run the net addition to
inventory is at the rate of PD per unit of time.
After Q units are produced (during Q/P units of
time) the inventory accumulation is (PD)(Q/P)
or (1D/P)Q. Defining the ratio (1D/P) as r,
then the TC can be shown, see, e.g. Johnson and
Montgomery (1974, p. 30), or Ca´ rdenas-Barro´ n