E-commerce products sale estimation and popularity

I need someone who has experience with
merchandise planning, optimising stock levels and sales performance.

BACKGROUND:
I work for a merchandise e-commerce store business, and I have control over how many products to sell in our e-commerce store.

As an example, we sell 200 bikes of a leading brand and they all sell, the problem is, I have control of selling more but not sure how many more to sell of this brand and still be in profit.

This will require an excel sheet with logic and formulas included, which I will need assistance with.

DELIVERABLES:
Excel sheet which contains a calculation formula that:
- Provides a potential product sales forecast which takes into account existing product sales volume for a given product range.
- A new column addition which will store data values for new sales volume of the product ranges.
- The output calculation needs to provide some sort out estimation of roi if we were to procure a higher volume of products and est. sale ability based on existing product conversion rates.

AIM OF SPREADSHEET - OBJECTIVE:
If my company orders 600 more bikes of a branded bike that is performing well sales wise, let's say we currently buy 200 of these branded bikes and receive 200% ROI. We want to workout if we purchase an additional 600 of these bikes what the ROI would be (based on existing ROI and online conversion rate of these particular branded bikes).
Here's an example of potential columns that could be included in the excel sheet:
Column 1/ Product Names
Column 2/ Order Volume (of column 1)
Column 3/ Cost price (of column 1)
Column 4/ Retail price (of column 1)
Column 5/ Product conversion rate
Column 6/ ROI of (column 1)

^ this is just an example, I'm open to other approaches, the logic provided above may not be correct, so I'm looking for someone to guide me on the best approach to achieve the required outcome.