This is exactly the problem. On portfolio margin IB sets the margin requirement according to their risk model and not according to some fixed rules. Meaning every new positions margin is depending on which positions are already open. So using WHAT-IF orders simply doesn't work for positions
build from many legs.
Instead of using WHAT-IF orders I now calculate the margin according to this rules:
Most often the margin requirement of the whole position is <= the sum of the legs according to the rules. It only happened once so far that the requirement was higher (probably there was too much concentrated risk).
Despair