I'll mention here the reasons for picking an Asset Allocation and sticking with it. In some recent threads I've seen, people change their asset allocation based on their predictions for the future securities market...it's much more likely you'll loose with market timing.
Stocks and Bonds serve as the prototypical categories for uncorrelated assets, meaning that changes in the market price for one category do not influence the price in a different category. History tells us that stock prices change much more (and more often) than bond prices. In those times when stocks drop dramatically, bonds might also drop, but a much smaller percentage.
One can spend considerable time to decide on the optimal asset allocation for oneself. One place where you can read about a thorough model to choose an asset allocation is here:
https://www.aacalc.com/about
The reality is that getting the best asset allocation is not important. It is most important to choose one that is good enough. What is good enough? One that you will not change based on what you see or read in the news, or most any other information you come across. It can be prudent for pre-planned changes with age, or based on big changes in overall financial situation (not just from market price changes).
It's a good idea to look at several online AA calculators, and then pick one. I've picked the following AA for my Thrift Savings Plan (TSP--the 401k for federal govt employees)
25% G fund (govt securities)
25% S fund (small cap index fund)
25% I fund (international fund)
25% C fund (large cap stock fund)
The impetus to do something when Mr. Market seems to go crazy is very strong. I admit I still can barely resist, but by having a good enough AA I always have something to to that is certain to be prudent--Rebalance.
If stocks tank, and I can't wait to do something, I rebalance. That is, I sell G fund (which doesn't go down--but grows very slowly) and buy the stock funds that are at a lower price relative to what they were the last time I bought them. My overall balance has gone down---but I got some depressed assets at lower prices. Let some months or years go by and then the stock funds have likely appreciated, they might grow to be 78-80% of the total. Then it's prudent to rebalance, sell some appreciated stocks and lock in those gains by buying some G fund.
Notice that whatever the market does, by doing only one thing, rebalancing, I always buy low and sell high, relative to the average price.
EDIT to add: with this approach, the particulars of which funds to use come down to being lowest cost, a small collection that are uncorrelated, and having an easy & low cost way to rebalance.