How to Fetch the Total Amount of the SalesVoucher

i am trying to calculate the Brokerage.
but when using the $Amount function.
it get updated as soon as i entered the first item in sales voucher . On adding another item, it is not getting updated.

Please help me admin

i am using the below code

[#Part:EI Details]
Add:Bottom Part:Brokerage Amount

[Part:Brokerage Amount]
Line:Brokerage Amount

[Line:Brokerage Amount]
Field:lblBrokerageAmount,txtBrokerageAmount

[Field:lblBrokerageAmount]
Use : Middle Prompt
Set as:"Brokerage Amount :"

[Field:txtBrokerageAmount]
Use:Amount Field
Set as:$Amount
Storage:BrokerAmount

 

please help admin

 

I tried to use

On:focus: yes: set field: $Amount

But when i trying to change in amount it is getting stuck in some infinite loop

Pleaae please help some one

 

After so much trying i finally got it

i have to use
Set Always:Yes