If you stick close to the recommended target watts all three buckets will fill in close proportion to each other.
Above TP all three buckets are filled in different proportions.
If you are short on High ~175% TP will fill that bucket the most.
If you are short on Peak a few sprints are in order (whatever you can muster at the time).
Outdoors you may decide to do the intervals later in your ride and accumulate more Low XSS during an extended warm-up or additional Low during a long cooldown.
The higher the CL you can reach (and maintain on successive intervals) the shorter the interval count. You can reallly beat yourself up if you want and shorten the ride. 
Time remaining is simply an estimate. I use it to decide whether to modify my route on the way back in.
If you don’t see “Update available” on Garmin Connect you are running the current release.
I try to keep this post updated with latest version number – Some helpful Magic Buckets guidance - Xert Community Forum