counting number of entries

I have a spreadsheet which contains a list of drawings recieved and I need to know how many unique drawings there are. The list of drawings is on one sheet with a summary page on a 2nd sheet.

Using the following works

=SUMPRODUCT(1/COUNTIF('Document Review'!M6:M129,'Document Review'!M6:M129))

However because the list is constantly growing I want to find a way of having the range to be automatic, or at least semi automatic.

Have tried entering the first and last row in cells c3 & c4.

I know that this works
=COUNTIF(INDEX('Document Review'!M:M,C3):INDEX('Document Review'!M:M,C4),"*")

but this doesn't

=SUMPRODUCT(1/COUNTIF(index('Document Review'!M:M,c3):index('Document Review'!M:M,c4))

Can anyone help - please

Try using Dynamic Ranges

(Entire columns won't work for Sumproduct in pre 2007 versions)