To evaluate a process at the early stage of its development, one must be able to perform simple calculations to compare all the alternatives. This must be done on a reasonably realistic basis, so that one can make credible decisions; however, the process should not be so detailed and laborious that it becomes too "expensive" to perform for alternatives that can ultimately be discarded. A methodology for performing these calculations has been developed, and this will be illustrated with a case study on Fischer-Tropsch synthesis (FTS). The effect of designing a Fischer-Tropsch (FT) process targeting a particular value of the probability of chain growth (alpha) on the overall carbon efficiency has been studied using simplified FTS flowsheet models. Two process configurations-namely, the once-through and recycle processes-have been compared, and it is observed that, for a fixed production rate of liquid fuels at 100% CO conversion, the carbon efficiency for the process with a recycle stream is higher than that of the once-through process for all values of alpha. However, if the aim is to maximize diesel production by hydrocracking the waxes, it has been determined that an optimal alpha value should be sought to reduce the cost of hydrocracking very heavy waxes. The incorporation of wax hydrocracking in the two processes reduces the carbon efficiency at all R values beyond similar to 0.7, thereby making it uneconomical to produce very-long-chain hydrocarbons.