This page finds the peak 20-day rolling average of VTI over the past 10 years (smoothing eliminates false peaks from volatility), and takes 95% of that peak as a threshold. It then compares the most recent closing price (from yesterday) to that threshold. If that close is below the threshold, the market is considered "down".
Always use a limit order when selling. Even if this page shows "MARKET IS OK", you can get sudden market drops.