Progress for dynamic Task queue

gistfile1.as

//Users can add tasks at ANY time,
// but they are only removed after they are finished
var tasks:Vector.<Task>;
var currentTask:Task;

function nextTask():void
{
    //Remove the first task from the list
    currentTask = tasks.shift();
}

function get progress():Number
{
    var numTasksRemaining:int = tasks.length; // Does not include the currently running task
    return (currentTask.progress + numTasksRemaining) / (numTasksRemaining + 1);
}

How would you create a "progress" property for a task queue that is constantly added to, and removes the finished task when complete?

Assuming each task has a "progress property" (with Number 0 to 1) this snippet could work, and will update very well when new tasks are added. But if the "currentTask" is removed from the list (decrimenting 'numTasksRemaining') the progress bar will jump back and forward rather oddly.

(in this example, "numTasksRemaining" does NOT include the "currentTask")

Any suggestions for a better solution?