Thanks for sharing the screenshot. Sorry, I am not familiar with Final Surge but check if there is any metric similar to Pa:Hr (Decoupling). If not, you can try this –
Calculate Avg. Pace: Let’s say it is 6:20 min/km (380 seconds/km)
Calculate Avg. HR: 138
Pace to HR ratio for first half = 380/138 = 2.75
Calculate Avg. Pace: Let’s say it is 6:42 min/km (402 seconds/km)
Calculate Avg. HR: 140
Pace to HR ratio for second half = 402/140 = 2.87
Pa: Hr for the run = % change in Pace to HR ratio from first half to second half = (2.87-2.75)/2.75 = 4.3%
For outdoor runs, when the pace is likely to change you will have to take into consideration the change in Pace as well to calculate the heart rate drift. So in the example you shared, drift is likely to be more than 1%.
Check this Training Peaks article for a better explanation of the metrics.
If the drift is less than 5%, then you can use the initial heart rate at the beginning of the test (likely around 138 or lower) as the AeT.
Hope this is helpful.