Parsing 何を考えているかすぐ解るような態度は改めたほうがいい > ... > > > > > > > > …… **** > > [Shiki-sama, that thing I said just now...] "Please do not tell it" Hisui mumbled. [What, that thing now you mean it's something I should not tell Kohaku. I see. I understand] That thing, I have to tell kohaku about it. [Shiki-sama. Ehm, what you are thinking I immediately understand, an attitude like that I think you should change it] [What? I am not thinking anything though] * * * I am not sure about > …… Is my translation correct? The fan-translation is: > Shiki-sama. I think you should try not to show what you are thinking. I am really not sure which one is correct, they are completely different.

The correct way to parse this is ….

The thing that he should is the whereby . That is, he should do something about that demeanor of his where anyone would immediately know what he is thinking. The grammatical content of the fan-translation is essentially correct here.

Your translation has all the right components, but it puts them together in a sort of incoherent way. In particular, Hisui is not directly saying that she knows what Shiki is thinking (though it's implied, obviously).

* * *

Addendum: I can't quite tell from your translation where you're getting tripped up on translating this, so maybe this breakdown will help.

* = "what [Shiki] is thinking"
* = "for [someone] to immediately know what [Shiki] is thinking"
* = "an attitude/demeanor whereby [someone] immediately knows what [Shiki] is thinking" (let us call this entity "X")
* X = "to alter X"
* X = "it is better to alter X"
* X = "I think you ought to alter X"