The cost of bringing someone on is compared with the contribution each sale leaves, not with gross revenue. To decide, include the full expected cost of the role.
An example with numbers
If the role costs $36,000 a year in total and you sell a service that contributes $300 after its variable costs, you would need 120 extra services to cover that cost: ten a month on average.
Try it with your numbers
Get the employment cost from a reliable source for your country and add other incremental costs. Compare the sales needed with the capacity and demand you expect.
This calculator is on its way. In the meantime, you can do the calculation on paper or in a spreadsheet.
Example figures for learning purposes. They are not a recommendation and do not replace professional advice.