Count of related records a few relationships away

So my brain is just not working today…

I am trying to get a count of Customers from a table a few relationships away

SalesLines – Sales – AccountNumbers – Customers

I have a subsummary report based on the SalesLines table which I am performing a find,
Product
Date Range (Sales)

I need a count from the found set of Customers, one Customer can have multiple account Numbers (that table contains the customerID)

A count of unique customer ID’s in the AccountNumber table would be fine.

I just can’t seem to get it to work.
If I am on the AccountNumber table, no problem I can use a summary field or an array of calc functions which all work well.
The problem is that I am filtering the list based on SalesLines & Sales.

Clearly there is a method that I have used in the past, but today it eludes me…

Any help is appreciated!