Basically,  I want to find an unusual day compared to previous 20days in this case.

If there is already a "canned" pcf or indicator  "bollinger" which captures this that is fine.

What is the best way to write?

RatioValue = (h-l) / (((H1-L1)+(H2-L2)+(H3-L3)+(H4-L4)+(H5-L5)+(H6-L6)+(H7-L7)+(H8-L8)+(H9-L9)+(H10-L10)+(H11-L11)+(H12-L12)+(H13-L13)+(H14-L14)+(H15-L15)+(H16-L16)+(H17-L17)+(H18-L18)+(H19-L19)+(H20-L20))/20)

One day price range  Divided by  S_r = SUM( H_i -L_i)  for i=1 to 20 ,  divide by 20

This is different than H-L / (avgH20 - avgL20)    ? correct.

What I am looking for is a Price range(change) Today which is a ratio  to previous 20Day price changes.

would be nice to have some functions like

=sum((H - L),1,20)

=sum(diff(H,L),1,20)   meaning summation of difference of (High,Low) for 1 to 20 bars ago

Such that  "RatioValue"  is large when (h-l) is large and S_r is small, 

    1 - indicates a normal day compared to last 20 days

     2 - indicates twice price range compared to last 20 days

etc.

Try this and see if the results look useful to you:

(H-L) / (AVGH20.1-AVGL20.1)

It uses the average high and low but should still show you large range days vs the previous 20 days.

I was looking for a case where this do not match, but I am having a  hard time.   So far, almost neglegable differences.

thx

(h-l) / (((H1-L1)+(H2-L2)+(H3-L3)+(H4-L4)+(H5-L5)+(H6-L6)+(H7-L7)+(H8-L8)+(H9-L9)+(H10-L10)+(H11-L11)+(H12-L12)+(H13-L13)+(H14-L14)+(H15-L15)+(H16-L16)+(H17-L17)+(H18-L18)+(H19-L19)+(H20-L20))/20)

Should be exactly the same thing as:

(H - L) / (AVGH20.1 - AVGL20.1)