Custom MKCircle like view outside the map view

Question

I wanted to implement a custom circle like MKCircle outside a map view and it should work exactly like MKCircle.

Basically when we zoom-in the map the circle gets bigger and when zoom out the circle becomes smaller. I need that formula to apply the same on my custom circle outside the map.

I was trying something like this:

let newWidth = (1-self.mapView.region.span.longitudeDelta) * 414
constraintCircleViewWidth.constant = CGFloat(newWidth)

but this is not accurate.