FMS Query for Calculation

Hi Experts,

I would like to write a FMS  query to allow for auto calculation based on the Inputs of 2 other fields before Adding the Sales Order.

For example. A query on Sales Order where I have Item ABCD  and Quantity 200.

I have a UDF named Abatement where the value is fixed 35%. And other is Assessable Value fixed with MRP Price say 350

And based on the result should be given in Assessable Value field.

Formula is  Assessable Value fixed  350 then Abatement  35 %  = 122.50 and then 350-122.50 =      227.50  should be displayed in the Assessable value field.

Kindly provide me the solution for the query.

Regards

Amol