"Unique Purchases" Metric

Hi everyone,

I'm trying to create a metric equivalent to "Unique Purchases" in Google Analytics. Would anyone be able to help me find the right calculation for this?

Unique Purchases is defined in Google as "total number of times a product was part of a transaction."

For example, if there's 1 order, for 2 products, with a total of 6 units; I'd be looking for 2 as "unique purchases".

So far, I've been trying to count the SKUs per order - but it isn't returning the right results. Any advice would be much appreciated!

COUNT (Product SKU exists (Orders))

Image may be NSFW.
Clik here to view.

Thanks for your help.