New Seller looking for a Good Way to Estimate GPM for Company HQ

I have been using https://sellercentral.amazon.com/hz/fba/profitabilitycalculator/index?lang=en_US but am having a hard time understanding inventory.

Here are some rough estimates:

Selling make 1,000 units/mo

Cost of Goods, $5.75 & MSRP of $21.00
Our fixed and variable costs are 0.65

I’m not sure how to estimate inventory maybe hold 1,000 and sell 500? Any advice?

These are only for generating an estimated net profit return for our HQ in South America. We make luxury bath products.

Thanks, we are new and this is overwhelming! Amazon sellers are so helpful.