I see that @v-piga-msft might have already given you a solution but since I'd already had a look at your file, I'll post mine too. Just in case it can be of help.
With the table structure and the columns that you currently have, I would place PlatformEntityName (and potentially SessionId under it, if you need it) and place the below measure in values. It will give you the average duration in minutes. You can change to Mins-Secs format from that if needed.
Hi @AlB , I did your calculation here and the average seems wrong, or maybe I don't understand something. In the example attached, we have a session length at 2 minutes 22 secondes, and the average on the session is 2.37 in your calculation; the average should be 2:22 as well (as we focuse on only one session). Do you know what I mean ?