I've recently started studying calendar spreads and I'm confused about something. Is it better to set up a calendar spread with options tending towards high implied volatility or low implied volatility? The way I see it, if you set up on a high IV, you collect more premium but you risk a drop in IV that could narrow your spread. If you set up on a low IV, you stand a better chance of your spread widening, but you collect less premium to begin with. How do you know which is best?