Total value in Group?

Hi,
I am setting up an old PC as a Ubuntu Plex / NAS server. I have a python program running on it that reports various stats using MQTT to HASS (Including 4 HDD free space). I want to create a group in my server group that will combine (sum up) all the free space values into 1, then by clicking on it - it will bring up the card with all the HDD separate.

What I tried is setting up a group for all the drives, then added this group to the group for the server. It does not sum up all the MQTT values though (but clicking on it brings up all the drives with values)…

What must I do to get all the values as a total?