Table that records Monthly Values of column in another table.

I'm new to quickbase so please bear with me.

I have a table (product details) for our company inventory.  One of the fields is a formula field called [inventory value].  This field calculates the value of fields [quantity]*[unit price].

I have another table (monthly inventory value) with the fields [recording month], [recorded value], [last updated].

[recording month] - should be a date field formatted to "MM-YYYY" or similar.
[recorded value] - should be the total sum of the column [inventory value] from the product details table.
[last updated] - date of last record change.

So, I'm thinking that I need to do a query on the products details table to get sum([inventory value]) of all records, then store the result value in a variable that can them be used in the monthly inventory values table.

Then I need to see if the record Today() (formatted as "MM-YYYY") already exists in the monthly inventory value table.  If it does, then edit record with new [recorded value] - sum of query.

If it doesn't exist, then create a new record with the updated "MM-YYYY" and record the values.

Hope this makes sense.  I currently have a Access Web App working like this using data macros, I just don't know how to make the equivalent functionality in Quickbase.

Mark