Calculating month movement and returning blanks

[JEB85]

Hi,

My powerpivot output is currently summarised as follows (the desired column is the result i would like):

FinancialYear	MonthName	Balance Sheet	Movement	Desired
2015	Sep	1,344	1,344
	Oct	1,570	226	226
	Nov	1,486	-84	-84
	Dec	1,526	40	40
	Jan	1,079	-447	-447
	Feb	1,480	401	401
	Mar	1,882	402	402
2016	Apr	1,051	-831	-831
	May	1,880	829	829
	Jun	1,549	-331	-331
	Jul	1,691	142	142
	Aug	1,740	49	49
	Sep	1,764	24	24
	Oct		1,764

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The 2 measures i have calculated are as follows:
Balance Sheet =CALCULATE(sum(BalanceSheet[Accrual (£)]))
Movement =CALCULATE(sum(BalanceSheet[Accrual (£)]),PARALLELPERIOD('Calendar'[Date],-1,MONTH))-[Balance Sheet £]

So i'm calculating the month movement on the balance sheet.
My data set begins in Sep-15 and will be updated on a monthly basis - the last set of data i have is Sep-16.
I would like the first month's movement to return blank (Sep-15 in this case) as essentially it's calculating current month - prior month balance sheet when there is no prior month's balance sheet.
Similarly, i would like to return a blank in Oct-16 as it's not a real value (i don't have any data for Oct-16).
The only complication is that my data set is at a product level and we have new products come in each month so if i pivot by product i don't want to return a blank if the product didn't exist in the prior month.

Is there a way to return the blanks i desire?

Thanks