Premade Running Total Measure

Hello, i used the quick measure function to help me create a running total which works great. However I want to get the running total to be 0 until we get to that month. So July to March should have 0 value as we have not got to that month yet.

This is the formula which quick measure generated - now as great as it is to make the measure for me i don't fully understand it to make any changes:

All RT = 
CALCULATE(
	[Total per month],
	FILTER(
		CALCULATETABLE(
			SUMMARIZE(
				'Date Table',
				'Date Table'[FMonth Num],
				'Date Table'[FMonth Name]
			),
			ALLSELECTED('Date Table')
		),
		ISONORAFTER(
			'Date Table'[FMonth Num], MAX('Date Table'[FMonth Num]), DESC,
			'Date Table'[FMonth Name], MAX('Date Table'[FMonth Name]), DESC
		)
	)
)

any help would be appreciated 

Abi