I'd go by feel. The main objective of the final mile is literally a warm-down, so on the one hand I often find the final mile of a tempo run is quicker than the first, just because I'm coming down from the quicker pace and the stride length is good, but on the other hand if you feel like jogging to get your breath back that's also OK. Hopefully over time you will notice not just an improved relationship between HR and pace at a constant pace but also HR coming down at a quicker rate during recovery.
I'd go by feel. The main objective of the final mile is literally a warm-down, so on the one hand I often find the final mile of a tempo run is quicker than the first, just because I'm coming down from the quicker pace and the stride length is good, but on the other hand if you feel like jogging to get your breath back that's also OK. Hopefully over time you will notice not just an improved relationship between HR and pace at a constant pace but also HR coming down at a quicker rate during recovery.