如何设置MKMap视图的缩放级别，使MKCircle（变化的半径）可见？

My map view contains one circle which covers many pinpoints with one center point.Whenever the user clicks a button the map should be zoomed in such a level the circle is visible.The map should not be zoomed more or less it should show exact circle.When user logs out then circle will have new radius in next login.How to calculate the appropriate zoom level. My code:My map view contains one circle which covers ma