For screening sequences of distillation columns, heuristic rules may be used, but the many contradictions and limitations of heuristics are well known. Vapour load is often used as a cost indicator for separation sequences. However, capital costs, which vapour load does not incorporate, also make a significant and variable contribution to the total annual costs of distillation processes. A "boiling capacity variable" has been developed, which is calculated from the number of stages and internal flow rates in a column. It measures the total internal vaporization rate in a process. That is, for a distillation column, the overall boiling capacity is the sum over all stages (including the reboiler and the condenser) of the molar vapour flow rate. The short-cut methods of Fenske, Underwood, Gilliland and others exist for calculating the reflux ratio and number of stages of distillation columns. The overall capacity variable can therefore be calculated using these short-cut methods. The optimal separation sequence for an ideal five-component mixture is investigated in this work. Distillation sequences are evaluated using both rigorous simulation with detailed cost analysis and the capacity variable and short-cut methods. The short-cut results are found to agree with the rigorous results, but have the benefits of requiring fewer detailed assumptions and being less computationally demanding.