How to get a tile representing a preview of a plan?

Question

Question is for the DroneDeploy platform.

By now my approach is getting the max difference in lat or lng, assign a zoom according to how big is the difference and get a tile:

..
var maxLat, minLat, maxLng, minLng, zoom;
plan.geometry.map(function(node){
    maxLat==undefined ? maxLat=node.lat : (node.lat > maxLat ? maxLat = node.lat : false)
    minLat==undefined ? minLat=node.lat : (node.lat < minLat ? minLat = node.lat : false)
    maxLng==undefined ? maxLng=node.lng : (node.lng > maxLng ? maxLng = node.lng : false)
    minLng==undefined ? minLng=node.lng : (node.lng < minLng ? minLng = node.lng : false)
})
if (maxLat-minLat > maxLng-minLng){
    if ((maxLat-minLat)*1000>1 && (maxLat-minLat)*1000<2){ zoom = 17; }
    // other scopes for zoom levels here
    else if ((maxLat-minLat)*100>1 && (maxLat-minLat)*100<9){ zoom = 8; }
}else{
    if ((maxLng-minLng)*1000>1 && (maxLng-minLng)*1000<2){ zoom = 17; }
    // other scopes for zoom levels here
    else if ((maxLng-minLng)*100>1 && (maxLng-minLng)*100<9){ zoom = 8; }
}

dronedeploy.Tiles.get({planId: plan.id, layerName: 'ortho', zoom: zoom})
    .then(function(tileInformation){ 
        const tiles = getTilesFromGeometry(plan.geometry, tileInformation.template, zoom); // getTileFromGeometry function take from https://dronedeploy.gitbooks.io/dronedeploy-apps/content/tiles/example-tiles-as-array.html
        console.log(tiles);
    });
..

1. Is there a better way to get a preview? When one get email from DroneDeploy about map being shared with you, there is a map preview embedded in the email. My guess that they use some undocumented APIs for that or the same approach which I described above.
2. It works only if I am the owner of the map, not in case the map is shared with me. I get using the returned path:

<Error>
    <Code>AccessDenied</Code>
    <Message>Access Denied</Message>
    <RequestId>774DCD7F4D31201F</RequestId>
    <HostId>
8dVjSI4yj/Ag3AJlV9NOZrZES8IXnCDp5kwmN7qjsoOX91+325FM6xlHUQ5QIsH9ZcMYTZwBym4=
    </HostId>
</Error>

Is there a workaround for such case?