How to calculate fiscal Last year YTD if the Current YTD sales only includes 3 to 4 months

[cwoo]

		Financial year ends april 30th
		last year	current year
		2016	2017
	may	900	100
	june	200	200
	jul	300	300
	aug	200	100
	sep	120	100
	oct	200
	nov	100
	dec	100
	jan	50
	feb	200
	mar	230
	apr	150
Year to date sept	1720	800
change   from last year	-53.5%

The  problem is calculating  last  year 2016  ( financail  year to date value )   "1720"    since the current  YTD  2017  ends in september  at  800.

When  use the  calculate function  and sameperiodlastyear  it  adds up 12 month  of last year not  may - sept  as needed.

Can any one help.

In short  we are just  trying  to find  our the  ytd  sales of the current year vs the year to date of the last year  "up to the current  sales month  in this case is  september .

Can anyone assist.

Colin