Next time you are making selections of areas in the Preview app on the Mac, try holding down the Option key to change up how selections work. Normally when you are making a selection with the mouse, the place you click defines one corner of the selection. But if you hold down the Option key, the click defines the middle of the selection area. So if you are trying to select an area around something, this makes it a lot easier to do so.

After your selection is made, you can also use the Option key if you are adjusting the selection. Normally grabbing a corner of the selection moves it. But if you hold down the Option key, it'll move the entire selection around the current center.