Formula to calculate the average weekly sales by quarter

Hi all,

I have a file with 3 tabs. 1st is the data tab that is populated by a separate system. Tab 2 is the Summary rollup by Quarter. Tab 3 is the fiscal quarter reference which shows which weeks should fall in which quarter.

I need a formula that calculate the average weekly sales by quarter. I need the formulas on Tab 2 and reference data on tab 1. I cannot have any formulas on tab 1 as our data system will wipe them out upon refresh of data. I was thinking it would be some kind of sumproduct that reference the week and tab 3 on which quarter it should fall within.

Let me know your thoughts on how best to achieve this.

Thank you,