The average disk space used this month is still incorrect and it will be until the feature to ignore gaps would be implemented
Until then you may use only the latest fully reported used space from the satellites for comparison. You need to make sure, that your node is received them from all satellites and they are complete (they should not differ too much from a previous report and the next one and shouldn’t be zero).
You also need to have a correct numbers on the piechart on the dashboard, they should match with the reported used space by the OS (you need to use a SI measure units (base 10) to compare). If they do not match you need to enable the scan on startup if you disabled it (it’s enabled by default), save the config and restart the node. After the used-space-filewalker would successfully calculate the used space for each trusted satellite and update the databases, the numbers on the piechart should be correct. This scan may take weeks (depends on the response time of the disk and its load).
You need also remove data of untrusted satellites: How To Forget Untrusted Satellites.
Then you may compare a difference between the last reported used space by the satellites and the reported used space by the node.