I have a solution in Siebel with the IIf (testExpr, expr1, expr2) expresion in Pre Default Value. I got some pseudo code
If Parent BC = BC1 or BC2, default = BC1/BC2.Field, else default = BC3.Field
but I don't know to write a right syntax. I tried with this, but I already know it's wrong :) :
Expr: "IIf (Parent BC = BC1 OR BC2,
default: BC1.Field/BC2.Field,
default: BC3.Field)"