Quote from riskarb:
It's approximately one-dev up and down, but based upon the implied vol, not statistical volatility. Run a 20/2 bollinger band on SPX and compare the stat-distro to that implied by the SPX atm straddle. The 20d[trading days] BB corresponds to a 30d straddle[calendar days]. You can manipulate the BB ma to fit the time to exp on the option combo. You'll see they are usually very tight.
Some charting/ananlysis packages will alow you to shadow-chart the atm combo superimposed on the spot chart.
Be mindful of the BB and/or the BB bandwidth when selling premium. It's a good visual indicator of option vols since stat and implied are [usually] very tightly correlated in index markets.