DE - Good fit. How about trying a logarithmic curve based on the 'compound interest' equation with a rate of 9%. Should also yield aprox the same shape and fit, with a 'real world' feel for the curve.
(Based on my observed 8 year doubling period, and the rule of 72.)
Just if'n youse in the mood to play with numbers a bit, and feel the urge. *s*