"Proportionate reasons" is the in-phrase this season. (Remember when it was "prudential judgment"?)
Each of us has reasons for everything we choose to do, even if it's just, "The coin came up heads." To justify a claim of proportionate reasons, then, it's not enough to merely give the reasons. The question rests entirely on whether the reasons are proportionate.
Which in turn raises the question, proportionate to what?
whether the reason alleged for a case of material cooperation [in a crime] bears due proportion to
the grievousness of the sin committed by the principal, and
the intimacy of the association with him
This uses the term "proportion" in a somewhat different way than St. Thomas does when talking about double effect:
Nothing hinders one act from having two effects, only one of which is intended, while the other is beside the intention. Now moral acts take their species according to what is intended, and not according to what is beside the intention, since this is accidental as explained above... And yet, though proceeding from a good intention, an act may be rendered unlawful, if it be out of proportion to the end.
This passage, written in arguing that killing in self-defense is not necessarily murder, is considered the foundation of double effect reasoning -- of which remote material cooperation with evil can be considered a special case.
Here, though, St. Thomas isn't concerned with the proportion between the good effect and the bad effect, which is the crux of the remote material cooperation problem. That proportionality comes for free when you've got a human life on both sides of the equation (or inequality (not strict)).
Instead, he's concerned with the proportion between act and intention -- to generalize, between means and end. We don't usually speak of that in terms of "proportion," which seems to suggest weighing two commensurate things (like two effects), but we can talk of proportion between cause and effect when an increase in one (such as force used in self-defense) leads to an increase (or decrease) in the other (such as incapacitation of an attacker). And once we get that foot in the door, we can speak analogously of proportionality between all acts and intentions.