W4 Extra Federal withholding option: Should this be on top of my regular federal tax amount?

I am talking about this area of the United States W4 Form:

"(c) Extra withholding. Enter any additional tax you want withheld each pay period: "

I have sold some properties in 2020 and I want extra federal withholdings to help cover my capitol gains. I changed my W4 to add $300 per pay check in this box (c) of the W4. My first pay check since making the change has EXACTLY $300 withheld instead of adding $300 to my normal withholding.

Is this correct, or is this a mistake on behalf of how my company is running payroll? I would have expected my regular federal withholding (let’s say it was $100) to be added, so it would be $400 in this example.