How to fetch subform field value to default field

I tried for that but if in 1st sub-form value updated with default field but
if i entered 2nd stock item then its fetch 1st stock item value (Sub-form)

How to fetch current stock item value ? please help me on this

Code:

[#Report:voucher]
    Variable:Lastsrno
    Set:Lastsrno:0

[Function:checkqty]
    Parameter:pCheckQty:Number
    20:Return:##pCheckQty

[#Field: VCHBATCH BilledQty]
    Delete:Switch
    Set as:$$AsQty:($$checkqty:(##lastsrno-1))
    Set always : yes
    Type : Quantity: forced   

[Variable:lastsrno]
Type:Number

[System:Variable]
lastsrno:0

 

Sir same result...as per first stock item entered that qty value fetch in 2nd item

 

[Collection: CurrentLineSr]
Data Source : Report : Current
Client Only : Yes
[#Field: VCHBATCH BilledQty]
Delete:Switch
Set as:$$CollectionField##Lastsrno-1):1:CurrentLineSr
Set always : yes
Type : Quantity: forced

 

If I remove -1 then on 1st stock item count line +1 and after 2nd item fetch value from 1st item

 

Problem solved....

 

Please close the thread with solution

 

see solutions for Amit sir reply