One method of determining the additional correction for presbyopia suggests leaving a percentage of the amplitude of accommodation in reserve. The rationale for this assumption seems logical because using all of the available accommodation is not sustainable without discomfort. However there is no empirical evidence indicating what percentage of the amplitude of accommodation should actually be left in reserve. Common figures adopted have been one-half and one-third. In this investigation the percentage of accommodation used is deduced mathematically after having determined the following: 1. The 'add' by the direct subjective clinical method. 2. Measured the amplitude of accommodation. 3. Measured the reading distance in 305 presbyopes ranging from 40 to 83 years of age. The results showed a small decline in the amplitude of accommodation up to the age of 52, after which age the measurements were scattered about a steady level. This finding suggests that after the age of 52 the results are based on the depth-of-focus of the eye. Females had slightly greater accommodation than males of the same age. The power of the add was significantly correlated to the age of the subject. The mean percentage of accommodation used for the 305 subjects was found to be 50.7%, thus confirming the rule of leaving half of the accommodation in reserve, although there were large variations: there were differences between males and females and with age the percentage of measured accommodation used, after having determined the correct add, diminished. Similarly the percentage of accommodation also decreased for shorter reading distances.