Inventory Days - Count back method

So i've made alittle progress.

I've added a measure to calculate a running reserver cumulative quantity - 

Reverse Cumulative Quantity = 
CALCULATE(sum('Purchasing Data'[quan]),
Filter(ALLSELECTED('Date'),'Date'[Date]>=MIN('Date'[Date])))

This shows the following:

This shows that 913.44 was purchased on 28 June 2019, and a further 996.48 was purchased on 8 May 2019.

My currently quantity in stock is 1,306.63, therefore want to report the date 8 May 2019.

However, my attempt has been unsuccessful:

Date_Target_Reached = 
IF (
    HASONEVALUE ( Stock_Record[stcode] ),
    VAR Stock_Quantity =
        [Quantity]
    VAR FirstDateOverTarget =
        FIRSTNONBLANK (
            'Purchasing Data'[deldate],
            IF (
                [Reverse Cumulative Quantity] >= Stock_Quantity,
                1
            )
        )
    RETURN
        FirstDateOverTarget
)

Any help would be greatly appreciated.