Need Rolling Average But Quick Measure Not Working

Hi everyone,

My team is trying to calculate a rolling average of quarterly numbers which will be used to calculate projections for upcoming quarters.  I've tried using the Rolling Average quick measure, but it just seems to be pulling the original value.  I can't seem to figure out what I'm doing wrong, so I'm hoping for a little guidance.  

I'd like to be able to take the average delta of the previous 2 quarters and project next quarter's performance based on this average.  If we were +10 in Q1 and -20 in Q2, I would expect to show a projection for Q3 of -5  (10 - 20 = -10 / 2 = -5).  Is this the correct math?  If so, how do I represent this in Power BI?

Here's some sample data based on my assumed math from above:

When I try Rolling Average Quick Measure, I get this in my visual.  If I drill down to the Quarter level, the "Max of actual" line is correct, but the "actual rolling average" line doesn't appear.

If I convert the measure to a column by copy/pasting the formula to a "New Column" formula, I can see the values it's calculating aren't correct.  They're just the values from Actual.

Here is the formula being generated by the Rolling Average Quick Measure:

actual rolling average = 
IF(
	ISFILTERED('DateTable'[Date]),
	ERROR("Time intelligence quick measures can only be grouped or filtered by the Power BI-provided date hierarchy or primary date column."),
	VAR __LAST_DATE = ENDOFQUARTER('DateTable'[Date].[Date])
	VAR __DATE_PERIOD =
		DATESBETWEEN(
			'DateTable'[Date].[Date],
			STARTOFQUARTER(DATEADD(__LAST_DATE, -2, QUARTER)),
			__LAST_DATE
		)
	RETURN
		AVERAGEX(
			CALCULATETABLE(
				SUMMARIZE(
					VALUES('DateTable'),
					'DateTable'[Date].[Year],
					'DateTable'[Date].[QuarterNo],
					'DateTable'[Date].[Quarter]
				),
				__DATE_PERIOD
			),
			CALCULATE(
				SUM('goals'[actual]),
				ALL(
					'DateTable'[Date].[MonthNo],
					'DateTable'[Date].[Month],
					'DateTable'[Date].[Day]
				)
			)
		)
)

I feel this issue is likely just my ignorance of Power BI in general, but I don't know what question to ask.  Is it a math problem or a "learning to use Power BI" problem?  Any guidance would be greatly appreciated.