  We derive a set of design guidelines and a figure of merit to aid the
engineering process of on-chip waveguides for strong Stimulated Brillouin
Scattering (SBS). To this end, we examine the impact of several types of loss
on the total amplification of the Stokes wave that can be achieved via SBS. We
account for linear loss and nonlinear loss of third order (two-photon
absorption, 2PA) and fifth order, most notably 2PA-induced free carrier
absorption (FCA). From this, we derive an upper bound for the output power of
continuous-wave Brillouin-lasers and show that the optimal operating conditions
and maximal realisable Stokes amplification of any given waveguide structure
are determined by a dimensionless parameter $\mathcal{F}$ involving the
SBS-gain and all loss parameters. We provide simple expressions for optimal
pump power, waveguide length and realisable amplification and demonstrate their
utility in two example systems. Notably, we find that 2PA-induced FCA is a
serious limitation to SBS in silicon and germanium for wavelengths shorter than
2200nm and 3600nm, respectively. In contrast, three-photon absorption is of no
practical significance.
