Asklemmy

There are a few levels of accuracy. Simplest is just using your max heart rate according to the equation (or trying to actually see how high you can get your heart rate), and basing percentages off of that.

Slightly better than that, most heart rate monitors/apps have some analytics built in that can factor in stuff like speed to approximate metabolic cost, and predict your lactate threshold. That's the heart rate that corresponds to the workload at which your body can't keep up with processing lactic acid (a byproduct of anaerobic metabolism). It's an important threshold cause you want some of your workouts to be definitely below that limit, and some to be definitely above.

There are ways to actually test that limit, often involving finger pricks to get blood samples while running on a treadmill.

The most accurate way (and what elite athletes will do), is a full metabolic test involving running on a treadmill with a heart rate monitor and a mask to measure oxygen consumption/co2 expiration.

For most people who just want to be healthy, and maybe get a little faster, it's not that important to be super accurate. The main thing is that in order to improve cardiovascularly, you basically need to activate the signaling pathways in your body that signify that you can't take in and process as much oxygen as you'd like to be able to. That involves high intensity work that is really hard on your body (muscles, joints, cardiovascular system) and it can take a few days to recover.

If you do most of your work in that low intensity zone, you give your body time to recover from high intensity while keeping overall volume up.

If you try to go too hard every time, you never recover, and never adapt.