I would recommend getting a chest strap that connects to your watch to increase the accuracy of your heart rate data. If you ever do a full VO2 max test and get your heart rate training zones, you can actually input that data into a lot of watches. That makes it more accurate because the watch knows 140bpm is in your zone 2, and if you’re now running faster at the same heart rate, you’re getting fitter. It’ll give you a dynamic estimate of your VO2 max based on previous predictions.
Another way that using a chest strap can be helpful is if you do a self-guided VO2 max effort test. That way your watch can pick up a new, accurate max heart rate. Everyone is so different—220 beats minus your age [the way most watches estimate your max heart rate] is a good indicator, but it’s a mathematical estimation, not a scientific measurement.