Another traditional source enumerating the criteria for double effect would be Don Curzio Nitoglia's article itself (though not a moralist, he certainly took these criteria from some traditional source):
"However, exceptionally, moralists add that this can be licit, only if four conditions are met:
1°) the act must be good in itself, if it is bad it is forbidden as sinful;
2°) the operating subject must aim at the good effect and not at the bad one, that is he must have a morally good intention and the bad effect must not be wanted or foreseen directly, but only allowed indirectly;
3°) the two effects must not be connected between them so that the good effect (end) is born from the bad one (middle), because you cannot do the bad to obtain the good, the good end does not justify the bad means;
4°) there must be a serious, just and proportionate reason to allow and not want the bad effect."