Calculate rate from related datasets

I have the monthly sales rate for various products. The products are sold in different countries. I'm looking for a meaningful way to calculate the sales rate at each country. The sales rate indicated below is across all countries.

Product     Global Sales Rate
Pen         9
Pencil      4

Product     Country Sold
Pen         India
Pen         Australia
Pencil      Italy
Pencil      Japan

When there is a new product launch, business team creates an opportunity including products similar to the one being launched. I know the sales rate country distribution for the products in the opportunity. I now want to derive the sales rate for each country (considering all products in the opportunity) and looking for a meaningful way to derive it.

The idea is that the new product will also follow the same sales trend across countries and the marketing budget can be better managed for each country based on this.

When business create the opportunity with similar products, there is no country influence. I want to find the sales rate for all countries mapped to products in the opportunity.

I had the below idea, but looking for better ways to calculate this.

Take each country C:
    SR1= Median sales rate calculated from products which are being sold in country C
    SR2= Median sales rate calculated from products which are NOT being sold in country C
    Sales Rate of country C = SR1 minus SR2

Some drawbacks in this approach are-

All the products within the opportunity could be sold in some countries (SR2 becomes 0), this might give some unfair advantage to this country. If SR2 SR1, then it can lead to negative sales rate for a country and this is meaningless. Can you please advise on a better approach for calculating the country sales rate for the opportunity?