I’m new to Tableau and was looking for a little bit of general direction please.

I have a product table joined to a date table using the release date field.

I’m trying to calculate a stock balance forecast for a product that goes on the market at a certain date (release date), at which point there’s an opening balance and each month the balance is depleted by X (rate of sale per month).

In the end, I would like a table with Product as the row, and each month as a the colomn, using the forecast stock balance as the mark.

The current system uses Excel and is all handed by a .net application. I think I could achieve this in PowerBI with Power Query, but that’s not available to me for this project.

I’m not even sure where to start; I thought I could create a left join with a data table and replace the nulls with 0’s to give me something I could use in calculations.

Perhaps I’m way off here, if anyone could offer a little advice or point me to some other resource, I’d appreciate it.