summarizing two child tables simultaneously

I have two child tables that I am trying to summarize into one report and perform calculations upon.

Both are many to one related to the parent. Other than having a copy of the key the parent is not involved in this report.

I want to Summarize(group) the report by the date(month) from one table, and sub-summarize the report by selected listvalue from the other table.

The table/record housing the date contains a number.

The table/record containing the selected listvalue contains a percentage.

In my report, I want to create lines containing calculations involving both number fields (for the moment, the calculated product of the number and percentage).

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Is the usual way to create reports composed of related child tables in FileMaker...

create a newcombo table containing all necessary fields together, so that even the wizard can easily create my report?

Then script a loop that steps thru the date records while sub looping thru the sibling table, creating new records in the newcombo table for each match.

Am I being really obtuse and missing a simpler solution?

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I have attached a view of the tables in my application.

The [PayableOrderLineItems] contains the date field and the number field to consider is named calc_lineTotal.

The [storeChargeLines] contains the storeCode (selected from valuelist - to summarize by) and the number field PercentOwned.

I want to create a report that is Summarized (grouped by) the month calclulated from DATE_to_applytobudget,

sub-summarized by the storeCode,

With lines that are calculated by multiplying the PercentOwned by the calc_linetotal.

Many thanks to all who respond.