In a nutshell, this is how I approach investing.
The Nasdaq-100 is the hundred largest companies on the Nasdaq, weighted by size, which in practice makes it a list of the biggest technology businesses on earth. Technology gets adopted, so the line runs up and to the right: roughly 14% a year since 1985. It isn't a perfect stand-in for any one company, but as a read on how the market is pricing technology as a whole it's as good as exists. The tool fits a trend line through that history and measures how far above or below it the index is sitting today. It's easy to overcomplicate how to time getting into the market; this makes discipline simple.
Fair means the index is sitting inside the band. Invest on schedule.
Expensive means it's above the band. Keep contributing monthly if that's your habit, but this is not the moment to deploy large amounts of capital.
Cheap means it's below the band. Historically the entry point, and historically the uncomfortable one, because the reasons it's down there are always splashed across the news.
I invest monthly. But whenever I want to deploy a larger amount of capital, I wait until the market is oversold. That is how the professionals do it: the further below its long-term trend the market falls, the more aggressive they get. The same dollar simply buys more down there.
One important caveat. The tool says nothing about when the gap will close, and nothing about any individual company. It tells you, broadly, where US technology is standing today. That's all. It's updated monthly.