[B9anders]

I'd probably eyeball "only on succesfull rolls" in the -50% region.

The main reason to take luck is to re-roll failed rolls and hence avoid at best unfortunate and at worst disastrous outcomes. This is the bang you get for the bucks from luck. Only being able to re-roll successful ones is to me a quite significant ***.

edit: Although "active only" is only a -20% modifier. "only on succesfull rolls" is still a bigger limitation than that. So -30%/-40% sounds about right.