HR isn't great indicator of rest for weight training. I do it by feel (or simply, by RPE). Usually, I wait first minute and judge whether I'm rested enough for the next set. The HR may drop within a few seconds but that doesn't mean you are ready for next set. Also, it depends on exercise, on the way you execute it etc. For some sets you will need more rest, for other less. Just find that sweet spot when you're rested but not cooling down. Watch won't tell you that. It's something you have to figure out yourself.
