I've studied the wiki page on glide paths, but am confounded by the variability in commercial options. The breakpoints and discontinuities seem artificial, though I liked the Morningstar curves. The log(100-age)-1 approach is interesting, but seems to roll off too quickly.

I'm also mildly troubled by the fact that the closed formula I've seen are not dimensionally correct. They have a hidden / assumed timescale. Using dimensional analysis to nondimensionalize the problem, and requiring that a formula transitions away from risk as you approach the time-scale of important fluctuations led me to the following:

X = asymptotic % stocks when very young (e.g. .8)

Y = asymptotic % stocks when very old (e.g. .2)

Tf = Target Retirement Age (e.g. 65)

T = Age

t = Characteristic time scale for market recovery (e.g. 10 years)

Pf = Target Retirement portfolio

P = Current portfolio size

R = Savings rate ($ / year)

**% Stocks = (X+Y)/2 + (X-Y)/2 * tanh ( ( (Tf - T) +**

*(Pf - P)/R*) / t )Note that the quantities in the Tanh are dimensionless. Tanh is an analytic approximation to a step function, who's sharpness depends on t, the market fluctuation / recovery timescale that you want to guard against. The italicized term describing dependence on portfolio size is optional.

The underlying ideas are that (1) there is a time before retirement at which you should begin de-risking the portfolio; (2) Your ability to take risks depends on the time to retirement (human capital); (3) Your need to take risks depends on your savings rate and how far you are from your goal.

This approach makes you more aggressive when further from retirement (either by age or by ability to acquire the needed portfolio), transitions as you near retirement, and then glides out to an end state. It is mildly counter-cyclical. This is reminiscent of what Sam Gamgee posted here. This formula is a combination of an age based rule and "% of enough" (see WCI post here, BH discussion here).

The general shape looks like this. This example assumes you approach the target portfolio evenly as you approach the target retirement age.

For this timescale (t = 10 years), this is close to the Morningstar glidepaths but transitions more quickly.

Comments? Advice? Like I said, I'm conscious that I'm over thinking it and that using a formula implies a precision that is illusory. I'm not using an existing commercial glidepath because my portfolio is spread across providers, all of whom differ.