Hi Louise,
Thanks for the additional information.
From what I know, you can not report on historical property values in HubSpot, only the current value of the property. So if a contact’s health score is 50 now and was 25 last month, we can only report on the 50 and can’t calculate an average between 50 and 25 UNLESS that 25 is stored in a separate property.
I think to make this happen you need to create a custom number field for each month of the year called health score Jan, Health score Feb, Health score March, Health score April and so on.
You would then need to export the health score property history and import it into these fields.
Then you could create a calculated property that finds the average of all these fields.
Best regards,
Jess