Turning Aggr Dimension into Expression

[#1]

What is the best way to turn Aggr Dimension into an Expression?

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My current dimension is Week, calculated as follows:

=aggr(if([Week] = min({$<[Product] = {'A'}>*<[Sales] = {'>0'}>}total <[Customer]> [Week]),[Week]),[Customer],[Week]

and my current expression then count(distinct[Customer])

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Now, I want to calculate the # of "new customers" on the fly in the expression, where I can choose any dimension and not limit it to observations. To make this possible, I would need to limit in the expression the Week=Max(Week) available in the database.

Ultimately, what I'm trying to accomplish in plain English : How many New Customers appeared in my database as of the most recent week?

thank you in advance!