I can only go by feeling. I can go as hard as I can on flat for 30-60 seconds. My legs give out first. I can get going so fast and try to keep up 30 or so MPH on flat for so long before I can't utilize my legs to keep it up any more. I don't know what my heart rate is doing. I can only judge what it feels like by comparing it to riding a hill.
Riding hills, so far there isn't a hill I can't climb, but I haven't tried any of the extreme grades in Pittsburgh yet (a coworker rode one of the dirty dozen hills last weekend and came back in shock over the steepness.) I am overweight and don't climb them fast, but on 16-18% I can tell you, my legs will grind up it slow but steady until I get to the top but my heart rate is screaming in my ears and I am dang near at my limit for breathing. I can tell, riding up a steep hill, it is definitely far more taxing on my heart rate and respiratory system than 30-60 seconds of all out sprinting on flat and the duration lasts far longer than doing HIIT sprints as it may take me 10 minutes or more to climb up a hill. Last big hill I climbed had 17.9% max, half a mile in length, and took me 15 minutes to climb up. That's the hardest I felt my heart and respiratory working since the very beginning when I started riding on the road with hills.