Google Map Circle Radius Fit To Map

I am facing a problem with a size(radius) of the circle to be fitted in map, considering the map zoom can be from 1 to ~19, you can see also in the picture below, in my case I have

Solution 1:

I found solution which counts the width/height of the view port of the map, and simply mad height/2 which would be the ideal radius of the circle

$("#create_zone_trap").click(function(){
    //Set the circle in the middle of the screen
    circle.setCenter(new google.maps.LatLng(map.getCenter());

    var spherical = google.maps.geometry.spherical, 
    bounds = map.getBounds(), 
    cor1 = bounds.getNorthEast(), 
    cor2 = bounds.getSouthWest(), 
    cor3 = new google.maps.LatLng(cor2.lat(), cor1.lng()), 
    cor4 = new google.maps.LatLng(cor1.lat(), cor2.lng()), 
    height = spherical.computeDistanceBetween(cor1,cor3), 
    width = spherical.computeDistanceBetween( cor1, cor4);

    //Fit the circle radius to the map 
    // 0.9 means 90% of the height
    circle.setRadius((height/2) * 0.9);

});