Try creating a named range for I20, I25, I30 etc. You can then use this named range in the RANK function.
1) Open the Insert...Name...Define menu item
2) Enter a name like MySubtotals in the the "Names in workbook" field
3) Enter a formula like the following in the "Refers to" field
=$I$20,$I$25,$I$30,$I$35,$I$40,$I$45
You may now use the formula:
=RANK(I20,MySubtotals,1)
Brad