Based on such a basic mistake, I wouldn't trust the rest of your comment. Everything you listed feels like made up arbitrary excuses. Most of the overhead per employee is fixed, that doubling rule of thumb is assuming an employee with a salary closer to the median. At the salary you're claiming, the overhead shrinks to something like 30% on top of the base salary.
If anyone took your numbers seriously they would run into the weird conclusion that an employee paid $50k actually costs a company $300k. That's the type of math error you're doing here.
The mistake of 'h' instead of 'y' typo?
Overhead is not fixed per employee - things like taxes and benefits scale with salary, not to mention the cost of tooling (hardware, software, AI token budget, etc) tend to be higher for higher-compensated employees.
You definitely shouldn't plan your business based on my comment, but you'd be foolish to ignore it.