A novel algorithm ("method of monodisperse growth") is described which has been derived on the basis of earlier work ("method of convolution", Gosden et al., 1995 and 1997a) and which makes the design of reactor control strategies more tractable by providing a very rapid route to a qualitative approximation of the molecular weight distribution (MWD) of products from living polymerisation processes. Numerically simulated data generated using this new procedure are compared with slower but more rigorous approaches to the same problem. The examples cover living polymerisations in an isothermal batch reactor, a steady-state continuous stirred tank reactor (CSTR) and feed-perturbed CSTR. It is demonstrated that, although the novel algorithm comprises only four differential equations, it provides the essential information concerning the position and relative intensity of the peak(s) in a MWD plot needed for the design of reactor control strategies for the production of tailored MWDs.