A 60:40 portfolio is designed to do that. The 60:40 has been analyzed to death and Monte-Carlo-simulated to death by retirement advisors.

With one health warning: most of the backtest era they're looking at had (a) much higher real bond yields (and equity earnings yields) than are currently available, and (b) covered time intervals of bond yields falling hugely on trend (and equity valuations rising on trend). Even if you beat a data set to death, if you're using data from situations that don't even vaguely resemble the current one, the predictions you get are not going to be useful.

Better to start with the basics: The most obvious conclusion from history is that long term returns from bonds are lower than those from equities.

A logical corollary: The only time you want any bonds at all is when the current relative pricing is such that it's one of the rare times that the prospect for bonds is higher. I bought some bunds in 2000 when that was true. This may be another such time, though my fixed income is very short duration.

The best single predictor of bond returns is the real yield on purchase date. Ten year US government bonds are yielding about 4.7% and USD monetary inflation is running around 2.4%, so that's a guess of forward real return of inflation + 2.3%/year. Five year TIPS are paying over 2%. Both are poor but positive. Still, it's a little more than I personally expect from the S&P 500 in the next 5-10 years, which is in the vicinity of the current dividend yield (i.e., index rising no more than inflation)

Jim