So you’re letterboxing the render target I assume?
That could be a bit wonky. I would guess you’d want to use the Rectangle’s Width and Height from CalculateRenderTargetPosition if that is the location of it in the window. But if you have other mouse requirements (such as you use the letter-box edge area or the letter-box edge area is just concealed area with valid world locations underneath - but only hidden, etc).
You’re probably off into specifics of your case land now.