i think i got it
http://www.option-trading-guide.com/bollingerbands.html now.
we need to plot the historical maximum expansion movement of the price with a reasonable look back period. so for example you want to see the maximum price moved in a year's time without breaching the upper band. if the price moved beyond the upper band then adjust the sd.
now you know the historically highest sd which Band managed to encampass the price. now use this as your sd value.
as a worst case scenario be ready to adjust the strangle by moving the winning leg closer to the price. you can move it till it becomes a straddle on the current expiry. if the trade is still in loss then roll over to the next expiry. you can even reverse the strangle by moving the winning leg beyond the loosing side.
this is not from my experience but based on optionalpha youtube video. please not am not affiliated to optionalpha, i am just trying to help.
strangle is race between (you) time decay and market (volatility). the trick is keep on buying more time till you beat the volatility.