Manual mainloop setInterval with locked framerate step

ml_manual.js

var interval;
// finishes when time-stepped 30fps reaches a virtual 20 seconds (600/30 = 20)
var ml = new CubicVR.MainLoop(function(timer,gl) { 
    // ...
    if (timer.getNumUpdates()===600) {
        clearInterval(interval);
        finished();
    }
}, true, true); // true 3rd param prevents mainloop from starting..

// step at 30fps exactly
var ml.timer.lockFramerate(30);
// run at 60fps max
interval = setInterval(1000/60,function() { ml.runOnce(); });

function finished() {
   // do stuff here..
}