Is that a fair way to analyse a 40/20 kind of session though? Because subtracting that 20-30 seconds suggests that each interval starts at a relatively rested state, whereas in the latter part of a 40/20 block, the 20sec rest interval is barely any recovery at all.
I take your point that people’s %maxHR at different intensities may differ. Perhaps it’s just a matter of finding your own baseline from experimentation, then use that to help you see which workouts push you hardest.