[darthtrader]

I think this is where you really want to learn some basic VBA if you really want to work with excel like this. I've been contemplating this but just haven't got around to learning it.
For a straight out spreadsheet it shouldn't be too bad though, just alot of columns.
Subtract A2 from A1 to get the 15 minute change in price. Once you have those make the columns for all the correlations and use the CORREL function , ie =CORREL(A1:A27, B1:B27). Once you have those make one more column, add the six correlation values across and divide by 6.