Need help with DAX Measure for Average Per Day

Okay, got it.
You can start by creating a general measure for your 'Good Leads' sum:

Total Good Leads = 
SUM('Historical Data'[Good Leads])

Then you can pass it into a measure that groups those sums and provides the average within the context of each unique date:

Average Good Leads Per Day = 

VAR distinctDates_TABLE = VALUES( Dates[Date] )

VAR result = AVERAGEX (
    distinctDates_TABLE,
    [Total Good Leads]
)

RETURN result

The result of this value can then be used by itself, such as on a card visual, to return the total average of good leads per date, or you can use a matrix visual to further group the calculation within the context of other categories, such as inventory type, placement, month, etc.