Fetching GST Rate details and pulling the Quantity from another form at the entry time

I am trying to achieve the below mentioned objective,

When entering a sales voucher, after selecting the Item, at the Quantity field, On pressing a key, I want to display a Form where the user will enter the Gross Total of the Invoice and the rate of the item, and that form will calculate the quantity for the invoice and i want that quantity to be taken to the original quantity field in the voucher.

Inputs:

1. Gross Total (Manual)
2. Rate of Item (Manual)
3. Rate of GST on the Item selected in the voucher (To be pulled from Tally DB)

Outputs:
1. Quantity

Problems I am facing:

I can't make it work to pull up the GST Rate details of the Item selected
Further, Don't know how to push the calculated quantity back to the quantity field on the invoice.

Please help!

 

Here is the existing code, please ignore purity field.

[Key: QCalc]
Key: Ctrl+X
Action: Alter: QCalc

;;for testing only
[#Menu: Gateway of Tally]
Add: Item: Before: @@locQuit: QCalc : Alter: QCalc
;;

[Report: QCalc]
Form: QCalc

[Form: QCalc]
Part: QCalc
Height: 20% Screen
Width: 30% Screen
No Confirm: Yes

[Part: QCalc]
Lines: QCGrossTotal, QCRate, QCPurity, QCGSTRate, QCQuantity

[Line: QCGrossTotal]
Field: Medium Prompt, QCGrossTotal
Local: Field: Medium Prompt : Set as: "Enter Gross Total:"

[Line: QCRate]
Field: Medium Prompt, QCRate
Local: Field: Medium Prompt : Set as: "Rate:"

[Line: QCPurity]
Field: Medium Prompt, QCPurity
Local: Field: Medium Prompt : Set as: "Purity:"

[Line: QCGSTRate]
Field: Medium Prompt, QCGSTRate
Local: Field: Medium Prompt : Set as: "GST Rate:"

[Line: QCQuantity]
Field: Medium Prompt, VCHBilledQty
Local: Field: Medium Prompt : Set as: "Quantity:"

[Field: QCGrossTotal]
Use: Number Field
Set as: 1000

[Field: QCRate]
Use: Number Field
Set as: 100


[Field: QCPurity]
Use: Number Field
Skip: No
Set as: 91.60

[Field: QCGSTRate]
Use: Number Field
Set as: $GSTIGSTRate:StockItem:##SVStockItem

[Field: QCQuantity]
Use: Number field
Set Always: Yes
Set as: @@QcQty



[System : Formulae]
QCQty : #QCGrossTotal * (@@GSTStdRate / (@@GSTStdRate + 100))

 

Getting the GST Rate details for selected item

$(Stockitem, $StockItemName).GSTDETAILS[1].STATEWISEDETAILS[1].RATEDETAILS[1].GSTRATE

Incase of state tax rate then use

$(Stockitem, $StockItemName).GSTDETAILS[1].STATEWISEDETAILS[1].RATEDETAILS[2].GSTRATE