lookupvalue on different table based on two filters

Hi,

I am trying to do a lookupvalue in a measure based on two filters in a different table. This lookupvalue is part of a much larger function with multiple ifs.

Here is a simplified breakdown of my tables:

Master tables (from database)

1) Receivables 

Ex: main columns: Month, Customer, Receivables

2) Invoices

Ex: main columns: Month, Customer, Invoices

Created tables

3) Calendar

- Columns: Date (budget year July - June by days), Month No., Rank (month no. based on budget year)

- This table is linked to both my 'Receivables' and 'Invoices' tables by "Month" to "Date" of 'Calendar'

4) YTD Summary

- Columns: Customer, YTD Receivables, YTD Invoices

What I am trying to achieve:

I want a measure of the # invoiced days overdue --> the simplified calculation for this is:

= (Total Receivables month 1/Total Sales month 1)*30 (# days)

There are many ifs in this formula that require a 'lookupvalue' for the sum of sales from previous month(s) (month-1 or I used rank -1). Not only do I want to grab the sum of sales from a previous month but for a specific client.

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I am able to achieve this lookupvalue measure based on the month alone:

Overdue table

I have another table for # invoiced days overdue:

- Columns: YTD Receivables, YTD Sales, Rank (month no. for budget year), invoiced days overdue, Month

# Invoiced Days Overdue =

IF('Overdue table'[YTD Receivables] = 0,0,

IF('Overdue table'[YTD Sales] > 'Overdue table'[YTD Receivables], divide ('Overdue table'[YTD Receivables],'Overdue table'[YTD Sales])*30,

IF((LOOKUPVALUE('Overdue table'[YTD Sales], 'Overdue table'[Rank],'Overdue table'[Rank]-1)>('Overdue table'[YTD Receivables]-'Overdue table'[YTD Sales])), 30 + (('Overdue table'[YTD Receivables]-'Overdue table'[YTD Sales]/(LOOKUPVALUE('Overdue table'[YTD Sales], 'Overdue table'[Rank],'Overdue table'[Rank]-1))*30,...,180 this continues for 5 months back and '180' is at the end.

In the simplified tables the #Invoiced days overdue is equal to 180; however, in my actual data there is more variation in the values. The bolded section of the formula is what I would like to create for my summary table but again, to filter by Customer and Month. 

I.e. if we are in August 2017 (Rank 2) client B, I would like my lookupvalue to sum the sales from Rank 1 for client B only.