Yeah but why would you? Usually if you start casting at a 10% HP enemy it won't be around for the 70% cast bar snapshot (which is arguably worse than a canceled cast regardless unless it messes up AF/Enochian). If you've coordinated with the group that you are the only one going to attack it, sure, but even then a F4 or even a F1 is wasted on a 10% HP enemy in that phase unless it's needed/the only thing to attack.