Standard deviation bands

[ianwaugh@xxxxxxxxxxxxxxxxxxx (Ian Waugh)]

Sums isn't my strong suit. Can some kind maths-type person help with 
this...

I want to plot a set of standard deviation bands, exactly like Bollinger 
Bands but using a different "central" average, let's call it MyAv. It 
takes two parameters - Length and Factor so I tried this:

Value1=BollingerBand(MyAv(Length,Factor),Length,+2);
Value2=BollingerBand(MyAv(Length,Factor),Length,-2);
Plot1(Value1,"BandUp");
Plot2(Value2),"BandDn");

But the bands are all over the place, often the lower band being 
*above* MyAv.

Can someone suggest what I'm doing wrong?

Thanks.

Cheers,
Ian