Hi,
I am running Volumio on a Pi3b/HiFiBerry MiniAmP with a 7" touch display. For dsiplay calibration 3 lines have to added to /boot/config.txt:
hdmi_group=2
hdmi_mode=87
hdmi_cvt 1024 600 60 3 0 0 0
Trouble is that after each version update (present version 2.671) these
For the moment you have to modify /boot/config.txt each time after the system has been updated. But in the future there should be a solution by using a file called userconfig.txt which will not be touched by system updates. For some more info have a look at https://forum.volumio.org/config-txt-overwritten-each-system-update-t10667.html and https://github.com/volumio/Build/pull/368. The PR with the necessary changes has already been merged. Maybe @michelangelo can give some hint when it will make it into a release 
Issue should be solved with version 2.673 which introduces /boot/userconfig.txt. Now you may add your configuration to this file instead of /boot/config.txt. 
When we merge the docs should automatically update… Will check why it has not
Just had a look at the docs: They still have not received an update