I've always been able to easily pass the 220 less age amount without dying (touch wood).
It's used in gyms by instructors because having unfit people who join on new years day being carted off in ambulances is bad for buisiness.
I don't bother with a HRM any more I find it to distracting.
I've found focusing on breathing works best for me, if I start to get into oxygen debt I ease off, if not then push a bit harder, I can usualy get into a nice rhythm and just wear my lefs out!