I need to create bollinger bands in Numbers

I am trying to chart a performance margin on a chart. I tried to chart a shaded area around a moving average using plus an minus STDEV's but the problem is that the STDEV cant show a negative number and this moving average travels into negative territory. Below is what i have.

for the moving average: =SUM(X815:X838)/25

For the top and bottom areas for the shaded area: =AC839+STDEV(AC815:AC838)

=AC839-STDEV(AC815:AC838)