At that age it's not so much the clock time, but the rituals that go with bedtime. So you probably can get 30mins buffer as long as you have the rituals. Some 3/4yo only need 11 or so hours others 13+ hours, so its more about knowing your child and having the firm rituals in place to show it's bedtime. So if you know you've had an active day with lots of outside time you start rituals a bit earlier, or a slow quiet day you could start them later or throw in some active time before bedtime rituals, such as rough and tumble play or dancing or make it a slightly later one with a Family movie night. It's more about having the rituals that help a child (either under or overtired) to relax. Often they can be a mix of over stimulated overtired, second wind making them seen undertired, but generally if they get lots of physical play, it's rare to be undertired, more unable to unwind due to overstimulated and so need the relaxing rituals of things like baths, cuddles, chats and being read books to.