My take is that there really is no set formula for determining things like that, as it's more getting a "feel" for what's happening in the markets. What's the market sentiment? Is there overriding pessimism or optimism occurring? If you can answer that accurately, then trading gets a lot easier, as you simply look for entries in line with that sentiment.
Watching the stock indices is one way to get this sense of the markets, but in my opinion you also need to be reading some news as well. You might try having Forex Live up while you trade: Forex News by Forex Live Read through just the headlines to get a sense of what's up. You can also use Talking Forex for a live audio feed: Talking Forex - First for FX News
As has been mentioned, determining aversion or appetite is probably as much art as science. However, re. your question, I usually look at % gain/loss in the indices rather than points. How much constitutes risk aversion/appetite? Again, no set answer for that, but obviously a 2% move in the index for example is more significant than a 0.50% move, etc. Anything beyond a 0.50% move I'd consider at least somewhat significant. These days it's nothing to have indices move 2-3% in a day.
You might consider just sticking a moving average on your indices charts to get a sense of trend also. I like to use a 50 EMA.
Personally though, if I don't have a good sense of what the sentiment is, I either trade conservatively, or I stay out altogether. So for me, a truly flat market is a time to stay out. But possibly Pip Dog might have some other comments about flat markets.
When risk aversion/fear is gripping the markets for example, it's pretty darn obvious. So I prefer to trade when the scenario is fairly clear one way or the other. If I really have no clue, I'd rather just stand aside until it is clear.
I don't know if I can tell you what constitutes a "Flat Market"...which really never is truly the case...more of a relative term. But, when I am setting up for a EUR news trade, for instance, I will not take a long EURUSD position if the EURUSD is down a 100 pips in the last hour due to stocks being down. And, if I recall the night of the UK CPI, the previous US stock session was down, Asia down, the Euro markets were down, and US futures were down. In this case, there is simply no way I'd be buying GU, EU, AU...which is the same as shorting the USD. Instead, I'm looking to either sell (spike) on a bad news report, or, as in this case, wait for a good report, and get ready to sell after the GU spikes up. It's a fairly good bet, as long as stocks don't reverse, that the GU would fall...sometimes quickly, sometimes over the next several hours. In the case of the UK CPI, it has a consistent track record of reversing the spike anyhow...which makes the "sell the spike up" strategy even more attractive.
But, in general, I like to see any of the FX pairs that I am setting up on for a spike news trade, to be fairly flat...quiet...low volatility...essentially waiting to react to the news.