applying st. dev. of a data set to a trendline forcast
 

I have a "bowl" with a capacity and flow through (water treatment plant). I
have a flow data set with a standard deviation (I cannot have for the entire
data set on the chart since it would turn black from too much data). I have
a line for 90% capacity of the plant. I have a trendline for my data set
(some data is already outside my 90%). I would like to apply that standard
deviation to the trendline to forcast when my plant can no longer handle the
flow (when my st. dev. is above my 90% capacity.