I have been doing 4x8 with reduced time between intervals for 4-6 weeks at a time. The plan is to get to 2-mins between intervals, so there is a 4:1 ratio of work:recovery at the end of the 4-6 weeks.
Intensity varies anywhere between 90-105% depending on how I feel. Good days will be slightly closer to 105% and bad leg days will be closer to 90%. HR doesn’t go above LTHR and all 4 intervals need to be the same, i.e. average power, and minimal cardiac drift.