I think everyone has gone away from this specifically (trying to keep the heart rate under a certain threshold). Keeping HR under a value is meaningful for steady state work, but is not meaningful for power repeats. It also can incentivise the wrong tendencies; i.e., not giving the work set a full, explosive effort.
What is meangful: the rate of HR recovery after the work set, keeping the trend in a session to where you see the HR recover down to somewhat of a baseline as opposed to continuously rising as the session goes on, and the trend over time as you continue to train this way -- being able to do more work at a lower HR.