Very good question. I have no idea!!!
More accurately, I have ideas but insufficient expertise to advise anyone. Again, I share thoughts while waiting for some wiser commenter to intervene.
Historical variance is good for proving things that can happen. Maybe not so good for defining the boundaries of how far variance can go. (Consider the book Black Swan by Nassim Taleb.) Maybe an additional strategy to prepare would be consider scenarios where a major event occurs, and seek asset allocations that would account for them as well as normal times. For example, if the euro collapses, that would be outside of historical experience in the PortfolioCharts data, but presumably USA portion of your allocation would be helpful at such a time. Likewise a rebalancing strategy would allow you to harvest value if you could execute it calmly.
One advantage of PortfolioCharts is that it provides data showing how different types of investments balance out a portfolio allocation, not just balancing stocks from different regions. For example, I found that according to the data, it appears that a surprisingly large amount of cash can be included with little or no harm to return as long as it is counterbalanced by a similar amount of commodities. I don't mean to recommend this allocation specifically, but rather am mentioning it as an example of different asset classes where maintaining a rebalanced allocation across asset classes could provide relatively stable returns in a wide variety of scenarios.
Fwiw, it appears in the PortfolioCharts data that the US environment is one of the few where stocks have consistently outperformed other asset classes. In more global environments, historically an all-stock allocation does not appear to be consistently best. But those data are only a few decades long. History has more examples but they are tough to quantify. This leads me to seek a somewhat diversified portfolio - perhaps the most diversified portfolio that does not calculate a reduced return, on the principle that diversification can address unexpected events in addition to expected ones - but I am learning too.
Please research carefully rather than just take my word for it!