Ranking values

Hi All, 

I have a list of sales orders and invoice values. I put them in a table to get a summary by part number:

When I try to use RANKX to rank those values it only works if I have the data un-summarised, so for each individual order that makes up the number in the invooice value column. How do I rank these summarised values so I cane se 1,2,3,4 etc descending.

This is what I have tried:

sales amt rank = RANKX('Final Sales data','Final Sales data'[Invoiced Sales],,DESC)

and it give me this:

I need a rank for each summed value by material number.

Thanks,