i've really found over the year that the best way to use HR is to use it in conjunction with RPE, and to more or less zero it at the beginning of every workout.

in other words, if maintaining low z3 feels harder than usual at the start of a workout, go an find an exertion level that normally feels like low z3, note the HR and kinda recalibrate around that...

i'd qualify that if youre off by more than like 8-10 bpms, theres probably somethign wrong, either youre tired or dehydrated or something...

thats strictly my rookie 2 cents, but its worked well for me this year.