[Shashi Shanbhag]

I think you are referring to this article –

https://evokeendurance.com/our-latest-thinking-on-aerobic-assessment-for-the-mountain-athlete/

Here is the training peaks article on Pa:Hr (Aerobic Decoupling used for outdoor tests) that also explains the calculation

https://www.trainingpeaks.com/blog/an-introduction-to-more-trainingpeaks-metrics-part-two/

For the heart rate drift (indoor test) calculation, instead of just taking two data points (beginning and ending HR), we are using the average heart rate of first half and second half as mentioned in both the articles above.

I would recommend doing the test again after a good warmup. If you want, you can make your workout public and share the link here.