for SPY, less than $200M is noise, more than $200M worth notice, more than $500M flow worth high-alert.
for IWM, not sure, didn't observe long enough.
sometimes, when SPY is not in the list but 大盘股 (XOM,CVX,JPM,BAC,GE,GS etc.) are all showing big ($150-200M) outflow or inflow, that's also worth notice.