Hi,
Javascript can do this easily, it should be something like below, you could need to do some changes, you can use this code onchange or your income field
var Value=Xrm.Page.getAttribute(FieldName).getValue();
var Amount=Xrm.Page.getAttribute(FieldName).getValue();
if(Value<=20000)
{
var Per=(Amount*15)/100;
Xrm.Page.getAttribute(FieldName).setValue(Per);
}
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