How to convert Cesium.Cartesian3 to Cesium.Rectangle

I have a bunch of Cartesian3s I am using as flyTos. I want the very first one to be the default initial view for the Camera. So I found this which says we can set Cesium.Camera.DEFAULT_VIEW_RECTANGLE before creating our View. That seems to work if I create an arbitrary Rectangle. But how can I convert my first Cartesian3 to a Rectangle? I don't see any appropriate function...

A Cartesian3 is a single point [x,y,z]. To make the rectangle you will need to provide additional information to your code.

Example is if you want the point to be the center of the extent, than you need to know how big the rectangle should be. Then you calculate the min/max latitudes and longitudes to make the rectangle out of.