Evaluation of the possible range of reserves associated with a prospect is a key part of risk taking in hydrocarbon exploration. The challenge of presenting a range of geologically possible models for a range of prospect reserve estimates is addressed using Swanson's 30-40-30 rule. Swanson's rule defines the mean as 0.3P(10) + 0.4P(50) + 0.3P(90), and provides a good approximation to the mean values for modestly skewed distributions. Pragmatic and mathematical justifications for this rule are given. Applications of the rule to a historical field size distribution and a specific prospect evaluation demonstrate its efficacy in handling routine problems in hydrocarbon exploration, with particular reference to use with the lognormal distribution.