Allocations: stocks and risk-free

#1
Has anybuddy seen a magic formula that suggests how much to invest in a basket of stocks and how much in some risk-free asset?

I have this one:
(1+r)(M - r) / [ (S^2 + (M - r)^2 ]
where M = Expected Mean of annual stock returns
S = Standard Deviation of annual stock returns.
r = risk-free investment

For example:
M = 9%, S = 25%, r = 4%, then the above suggests 80% in the stocks.

I'm not looking for advice (I never use this stuff meself), but am interested in "other" such magic formulas.

The above magic formula can be found here:
http://www.gummy-stuff.org/pillow.htm
 

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