How do you decide on an asset allocation?

If you know what ETFs, stocks, REITs, etc you want to invest in, how do you decide what % of your portfolio to allocate to different investments?

For example, say you want to have a strong backbone of VTI/VOO in your portfolio, but you also want room for individual stock picks. What can we look at to decide if that allocation to the backbone ETFs is 25%, 50%, or 75%?

I am looking for a research/stats-backed answer preferably. Thanks!