deddiekated,
Indicator words are a good start to identifying parts of a conditional, but you do need to make sure that the conditional diagram makes sense of the sentence in the stimulus of which it's supposed to be the diagram.
In this stimulus, we have one of the two following things:
rational
justification forms essential part of explanation
or
justification forms essential part of explanation
rational
In order to determine which of these is the correct diagram of the conditional, and which is the Mistaken Reversal, paraphrase each:
"Rationality of an action requires its justification to form an essential part of its explanation."
or
"Whenever an action's justification forms an essential part of its explanation, that action is rational."
The former is the correct paraphrase of the stimulus, so the former diagram is the correct one. The stimulus is saying what rationality requires - only actions that meet these conditions are rational actions.
Compare that to PT 54, Section 2, #23, answer choice (A):
food
fridge
or
fridge
food
Either
"All food in Diane's apartment is in her fridge."
or
"All food in Diane's fridge is in her apartment."
The latter doesn't sound right - unless her fridge isn't in her apartment, wouldn't it be a trivial statement to say that all food in her fridge is in her apartment? Whether this is an obviously true statement or not, the point is that this doesn't match the first sentence of answer choice (A). Thus, it's not the correct paraphrase. It doesn't sound right because it doesn't match the statement made. So, the first diagram is correct.
Getting the diagram in the right order is essential, as the Mistaken Reversal is going to make the right answer look wrong, and potentially make wrong answers look right! However, you don't always have time to engage in a very lengthy process of diagramming twice, making two paraphrases, and checking each. To save time, when you think you have two things related via a conditional but are unsure which is sufficient and which necessary, ask which one requires the other to be true. The sufficient requires the necessary. This should give you the correct order of the conditional.
Robert Carroll