VO2Max estimate on Epix 2

The Epix 2 estimates VO2 Max for cycling after a couple of rides of at least 20mn

epix (Gen 2) - Getting Your VO2 Max. Estimate for Cycling (garmin.com)

The Firstbeat algorithm uses data it estimates normal, based on oxygen consumption based on heart rate and certain performance metrics (like pace or power for cycling).

One key data point to enter is your HR Max. I would start with using a standard formula like 220 minus your age. You can find out whether your VO2 max is well estimated by looking at your race estimates for running a 5K, or by looking at the pace goals for daily suggested running workouts of the VO2 or Threshold type. If these paces are too optimistic, your VO2 is too high for your age. First, make sure you run a balanced set of workouts so that the VO2 estimates can have the full range of HR/performance data. Second, if still too high, increase your HR Max by a couple of bpms. This will tell the watch that in fact you are not as capable of sustaining performance in the higher range of HR Max as previously observed, and therefore your fitness estimation will decrease.

I follow your reasoning up until the very last point; If the reported VO2 value is too high due to wrong MaxHR setting, the MaxHR setting should be decreased.

Reason for too high value is that the intensity (%MaxHr) is probably closer to the real max than what the watch thinks. 

Very clear, thanks. The only problem is that I cannot run for several reasons. Can I adapt this approach to cycling? Also, I have an MTB, so cycling could be on flat roads but not with with a road bike. 
Thank you. 

Yes, this is the same approach with biking, but you need a power meter, or your ebike needs to be able to share the rider-power data to the watch. Some ebikes do.

(+) Pairing eBike with Garmin 1030 - Edge 1030 - Cycling - Garmin Forums

If you can record rider-power with your activities, then you can focus on a balanced training approach. Just follow the daily workout suggestions. Your VO2 max estimates will improve as your cover all types of power-based workouts.

Otherwise, walking will give you a VO2 estimate, but without high heart rate performance, it might be less accurate.

Thanks for the clarification. I have a Rocky Mountain Altitude Powerplay C70, I'm pretty sure it doesn't stream power data. I look forward to have some change in VO2Max calculation, in order to work with thousands of riders using ebikes that don't share data.