Column C worked beautifully! Didn't take 2 minutes, so I'd done it before I saw your update.

But column B gives me a #Div by 0! error as soon as I drag the formula past the first 55 rows. I'm not familiar with either index or ceiling, but I'll research both of those to see what I've done wrong. Here's how I modified your formula in order to fit the top cell of my actual spreadsheet:

=AF2/INDEX($AH$2:$AH$44936,55+(CEILING(ROWS($1:1)/55,1)-1)*20)*40

Here's what it looks like in the first cell where it gives me a #Div by 0! error:

=AF57/INDEX($AH$2:$AH$44936,55+(CEILING(ROWS($1:56)/55,1)-1)*20)*40

Thanks again for your help!

- LCamille