Max heart rate?

I’ve not long finished the C25K so thought i’d treat myself to a Garmin fr 35 watch to keep track of my times and heart rate. I know there is an easy way of 220 - your age (which mine is just 58 😊) but is there another way to find my max hr?

I did a 2.5k run today outside (I usually run on the treadmill) and found it quite difficult.

Hr of 57 low, 180 high 😬 and a average of 160.

Now i usually run about 4.5k on the treadmill no problems. My pace might’ve been a bit faster than usual but it always is when i run outside 🙄

5.49/km according to my watch.

Thanks.