It struck me as being funny the way reader Jim phrased his question, although this very same thing probably has happened to many investors at one time or another:
If I had been asleep when my ETF crossed above its 39 wk M/A, what would you recommend as a late buy signal?
Actually, this happens in my advisor practice all the time, not on purpose but via the mere fact that new money comes in during the middle of a buy cycle.
As a general rule, I use my incremental buying procedure to ease into the market. I immediately invest 1/3 of available assets and increase that by another 1/3 once the original portion has gained 5%; and so on until I am 100% invested. That decreases my risk sharply of allocating 100% of a portfolio at a potential market top.
If you are not that conservative, use a 50/50% ratio or, if you are aggressive, go all in at once. Remember, if you combine this approach with my recommended 7% sell stop discipline, you already know what your potential risk will be.
Simply assess your risk tolerance and then invest accordingly. If neither of these three suggestions appeal to you, do the next best thing you can do for your emotional well being—stay out of the market.