It seems to me that what he means to ask is this:
Since all the excitement and desire are rooted in lack of attainment / lack of knowledge, if I have already seen an improper sight, and I stop now, then my mind will go on inflaming the desire to enormous proportions, and I’ll long for “mountain shadows as though they were mountains,” whereas if I let my desire have its way, I’ll see that there really isn’t all that much pleasure in it.

I wrote this only to explain the questioner’s view. Obviously, the commandments were not given in order to draw a person toward transgressions, and the same principle could be applied to most prohibitions that a person’s soul desires.

Indeed, as the rabbi commenting above said.
What I meant by “how far” is whether this applies only in circumstances like the story above, or whether even when the reason touches on thoughts of the transgression themselves, one may not go beyond the line.

Sorry if it came across as unserious.
Also, Elisha, your “obviously” is not at all obvious to me.

If this is your way of dealing with sinful thoughts, then in my opinion it is permitted. If you do not know whether it will help, then it is a doubtful prohibition.

By the way, is there actually a prohibition on sinful thoughts in themselves, or only as the most likely route to seminal emission?

https://ph.yhb.org.il/14-04-06/

So I’ll put it more simply.
We do not know the reasons for the commandments, and even if we did know them, we are not authorized to decide on the basis of our own judgment by calculating the desired purpose.
To the extent that thought is forbidden, whatever its reason may be and whatever the situation may be, who has permitted it to you?

First, it is not clear whether this is Torah-level or rabbinic. Second, the question is about the definition: is what was forbidden the thought in itself, or thought that leads to a transgression? Therefore the issue of “the reason of the verse” is not necessarily relevant here. Just as the Sages permitted seclusion in situations where there is no concern for a transgression, and did not say that we do not derive law from the reason of the verse. And so too in other cases. See also Ritva at the end of tractate Kiddushin, who says some nice things about this.

The truth is I was responding to the questioner; I don’t have the courage to argue with the Rabbi.
But since the matter has already been opened up, I’ll ask: even if we say that only thought that leads to transgression was forbidden, who appointed a person to distinguish between one thought and another? Even aside from the words of the Sages that there is no guardian in matters of sexual prohibitions, that is human nature: he thinks, and desires, and acts. In my opinion, a person cannot know what the outcome of his thought will be.

If you mean cases like “her husband is in town” and the like, it seems to me that the intent is not that there is seclusion here and the Sages permitted it because of some external reason, but that because her husband is in town, there is no seclusion here.

The distinction you made regarding “her husband is in town” is the distinction I’m talking about here: definition and not reason. The definition of the prohibition is thoughts that lead to transgression. A thought that does not lead to transgression was not forbidden (it is not the kind of thought under discussion).
As for a person’s own assessments of himself, I see no reason one cannot try to assess. One should do so carefully and with concern. See Ritva there. After all, you are suggesting being stringent out of doubt, but forbidding looking because of concern for thought is also a stringency, since you are telling him to remain with his thoughts and not look at the nakedness (so that he won’t think about it).