[amibroker] Triggering a Chandelier trailing stop with % favorable price excursion

["cohndw" <minimax9999@xxxxxxxxxxx>]

I am trying to calculate the % favorable price excursion based on the
current price vs the buy price, and then turn on a Chandelier trailing
stop if a % favorable price excursion threshhold is hit to lock in
more profits.  I am unable to grab the buyprice on the first buy
(opening price).  The code I am using is shown below.

OpenPrice = BeginValue(ValueWhen(Buy>0,Open,1));

Thanks,

Dave