I've never tracked my heart rate, I can only comment on how I feel when doing HIIT... I feel like I've climbed a moutain, lol.
It's hard work...but seems to have worked for me and many others.
I've been known to lay on the ground for a few mintues during HIIT too
And unless you go have yourself tested on a metabolic level you really don't know how many calories you're burning... 400 may be a good estimate I don't know... just putting that out there too. Your weekly and monthly progress via how you feel and if your clothes are feeling looser will be the ultimate judge
Never rely soley on the scale..the scale lies, lol. (water, muscle, fat... the composition of you may change greatly, but your weight may not...)