In my data source I have column Quoted and Submitted.
Dividing the Sum(Quoted) by SUM(Submitted) gives us Sub to Quote ratio.
I would try something like this:
Sub to Quote =
AVERAGEX(
SUMMARIZE(dim_Date,
dim_Date[Qtr Year],
"Average", DIVIDE( [Total Quoted], [Total Submitted] )
),
[Average]
)
This summarizes your table to get the average for each quarter and then averages those averages.
Note: This returns 8.15% rather than 8.18%. I'm guessing this is just a rounding difference though.