How do I put into formula the difference of the high of the 3 std dev of bollinger band to the low of the 3 std dev bollinger band? 

The following is based on Bollinger Bands 20, 3.00 and produces the value of the top Bollinger Band minus the bottom Bollinger Band. Which ends up just being 2 times the number of standard devations specified in the Bollinger Band (so 6 in this case).

6 * SQR(ABS(C ^ 2 + C1 ^ 2 + C2 ^ 2 + C3 ^ 2 + C4 ^ 2 + C5 ^ 2 + C6 ^ 2 + C7 ^ 2 + C8 ^ 2 + C9 ^ 2 + C10 ^ 2 + C11 ^ 2 + C12 ^ 2 + C13 ^ 2 + C14 ^ 2 + C15 ^ 2 + C16 ^ 2 + C17 ^ 2 + C18 ^ 2 + C19 ^ 2 - 20 * AVGC20 ^ 2) / 20)

Modelling Bollinger Bands (& Standard Deviation) in a TC PCF

Hi bruce I am trying to get the difference of the upper and lower 3 st dev bollinger band.

How do I test it to know if the values are correct? 

Hi Bruce, 

If the value of the upper bollinger band 3 st dev is 119.20 then the lower bollinger band 3 st dev is 118.70. The difference is .50. How do I make this a formula?  

What I mean is how do I make a formula for that? 

You should just need to change all instances of 20 to 30 and add 10 terms.

6 * SQR(ABS(C ^ 2 + C1 ^ 2 + C2 ^ 2 + C3 ^ 2 + C4 ^ 2 + C5 ^ 2 + C6 ^ 2 + C7 ^ 2 + C8 ^ 2 + C9 ^ 2 + C10 ^ 2 + C11 ^ 2 + C12 ^ 2 + C13 ^ 2 + C14 ^ 2 + C15 ^ 2 + C16 ^ 2 + C17 ^ 2 + C18 ^ 2 + C19 ^ 2 + C20 ^ 2 + C21 ^2 + C22 ^ 2 + C23 ^ 2 + C24 ^ 2 + C25 ^ 2 + C26 ^ 2 + C27 ^ 2 + C28 ^ 2 + C29 ^ 2 - 30 * AVGC30 ^ 2) / 30)