Asset allocation is the answer to "what percentage in stocks and what percentage in bonds?". Classic studies attribute most of the variability in a portfolio's results over time to that decision: far more than the choice of individual holdings.
An 80/20 portfolio and a 40/60 portfolio are not more and less ambitious versions of the same thing. They are two different experiences: the first can fall 35% in a bad year, the second around 18%. Both figures are normal, and the relevant question is not which returns more but which you can hold without selling.
That is why the right percentage depends more on your horizon and your actual tolerance, not the tolerance you would claim in a calm month, than on any market forecast.