Substract the next value from above of this by group by in a chart

[geogou1973]

Hello,

I have the following table 

Store	Date	Item	Inventory Qty	Diff		Diff Correct
021	24/02/2021	410856	251
021	10/03/2021	410856	135	-116		-116
021	19/04/2021	410856	220	85		85
021	17/05/2021	410856	350	130		130
021	16/06/2021	410856	186	-164		-164
040	24/02/2021	410856	542	356
040	10/03/2021	410856	373	-169		-169
040	19/04/2021	410856	60	-313		-313
040	20/04/2021	410856	385	325		325
040	06/05/2021	410856	13	-372		-372
040	17/05/2021	410856	154	141		141
040	18/05/2021	410856	264	110		110
040	16/06/2021	410856	337	73		73
021	24/02/2021	456734	156	-181
021	10/03/2021	456734	265	109		109
021	19/04/2021	456734	452	187		187
021	17/05/2021	456734	70	-382		-382
021	16/06/2021	456734	236	166		166
040	24/02/2021	456734	123	-113
040	10/03/2021	456734	235	112		112
040	19/04/2021	456734	90	-145		-145
040	20/04/2021	456734	423	333		333
040	06/05/2021	456734	65	-358		-358
040	17/05/2021	456734	321	256		256
040	18/05/2021	456734	456	135		135
040	16/06/2021	456734	245	-211		-211

I have the store , the date , the item and the inventory qty in a chart. In the column Diff i have the expression

SUM([Inventory Qty]) - ABOVE(TOTAL SUM( [Inventory Qty])) 

The result is not correct because substract all the values for all stores for all items one by one. 

The result i want to have is the Diff Correct that substract the values only by store , item.

How can i manage this in order to have the results of the column Diff Correct in a chart ?

Thank you in advanced.