Find a good position PnL simulator and play with it. Expiration PnL is pretty misleading for BWBs because unlike most other strategies, the curve doesn't just grow upwards into the expiration shape, it also moves horizontally, starting far to the left of the actual strikes. No voodoo here, it's simply because the verticals in BWB are asymmetric.
Here, take a look at the attached PnL for a SPY BWB (+184 -2x186 +190) open today in Feb 01'14 options. The white line is PnL halfway to the expiration on 1/17. See how at that time the maximum PnL is actually at about 183.3, below the nearest long strike. As time passes, that hump slowly grows and moves to the right. But even with a week to expiration it's still just past 184. You only start seeing something close the final expiration PnL in the last few days, both in terms of profit potential and the underlying price point.
Here, take a look at the attached PnL for a SPY BWB (+184 -2x186 +190) open today in Feb 01'14 options. The white line is PnL halfway to the expiration on 1/17. See how at that time the maximum PnL is actually at about 183.3, below the nearest long strike. As time passes, that hump slowly grows and moves to the right. But even with a week to expiration it's still just past 184. You only start seeing something close the final expiration PnL in the last few days, both in terms of profit potential and the underlying price point.
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