To the OP: I just registered to thank you. Using info from Garmin. I use a manual method and this was spot on.
@Hjenei, dividing the pace 362sec / 356sec = 1.68% but this is after 4 roundings to provide an average for denominator and numerator in both HR and pace.
So, your “151” in HR can be between 150.5 and 151.4 (and same applies for pace in H1 and H2)
Let’s pretend the paces are precise: 5:56.000 and 6:02.000 The HR rounding error will give you anywhere between 1.08% and 2.29%