How to Calculate Assessable Value in transfer order

Hiii guys,

I want to ask you guys just like in sales order, i want to calculate assessable value. For that i made a menubutton “misc charges”,for that i done the coding.As we know in misc charges form, when we enter the misc charges value and check the assessable value checkbox

calculation of Assessable value in transfer order form will be Net Amount + Misc Charges Value

and if we not check the assessable value checkbox

Assessable value = Net Amount

But when i created the transfer order, net amount is calculated but assessable value is not calculated. Can anyone help me to solve this problem ??