Personally, I (loosely) apply the law of diminishing return. The more you stack flaws into a single area (formulaic magic in this case), the less those flaws are worth. Until they are worth nothing. I find that it prevent munchkinism.
So, to a player who wanted to stack Rigid Magic and Unstructured Caster, I'd probably say "Sure, but count Rigid Magic as a minor flaw, because you've already eliminated over 90% of his ability to use formulaic magic."
The "order" in which you introduce the flaws have no relevance to me. It's the whole package that I evaluate.
Ah, that's a different perspective from both JL's and mine. You and I don't agree about the value of the package, but I think we do agree about the primacy of placing a value on the net effect.
That doesn't seem that clear to me, since in previous posts you have implied that the order in which the player took the flaws would change how you evaluate them.
Specifically:
Both of these seemed to indicate that the order in which the player has an impact on how you evaluate it. Now you say it doesn't. So which one is it, really?
I'm not trying to be snarky. Just saying that the way you present your position is a bit inconsistent.