A computer system for processing data relating to an investment arrangement having an account value dependent on investment performance and available for withdrawal by an account owner includes a data storage device storing data indicative of a present account value, a withdrawal factor value, a present payment base value, and a guarantee of payments available periodically for a term without reduction of the payment base value; and a processor. The processor is configured to, for periods after a first withdrawal, determine the present payment base value by comparing a present account value to a prior period account value, a periodic increase in the payment base value being limited to a maximum percentage; and determine an available maximum withdrawal without reduction of the payment base value based on the withdrawal factor value and a greater of the present account value and the present payment base value.