Transcendental Signified as the Basis of Deconstruction Theory

In his defense of writing in his book Of Grammatology , Derrida introduces some concepts that later became central to the deconstruction theory. The most important of which is the transcendental signified “If reading must not be content with doubling the text, it cannot legitimately transgress the text toward something other than it, toward a referent (a reality that is metaphysical, historical….” 1825. To explain the meaning of the transcendental signified with reference to the article itself as well as my previous understanding of this concept, I can say that Derrida assumes that the entire history of Western metaphysics from Plato to the present is founded on a classic, fundamental error. This error is searching for a transcendental signified, an “ external point of reference” ( like God, religion, reason, science….) upon which one may build a concept or philosophy. This transcendental signified would provide the ultimate meaning and would be the origin of origins. This transcendental signified is centered in the process of interpretation and whatever else is decentered. To Derrida THIS IS A GREAT ERROR because
1. There is no ultimate truth or a unifying element in universe, and thus no ultimate reality (including whatever transcendental signified). What is left is only difference.
2. Any text, in the light of this fact, has almost an infinite number of possible interpretations, and there is no assumed one signified meaning.
Accordingly, whatever pre-constructed meaning of a text has to be deconstructed to examine the possible different meanings. What is centered has to be decentered and what is privileged has to be unprivileged. Rather than providing how a text means ( like new criticism methodology), or discovering how a text means ( Structuralism methodology), Deconstruction aim at showing that what a text claims saying and what it actually says are “discernibly” different.
I have to say at the end, that Derrida requires eliminating whatever transcendental signified in the process of interpretation, but, at the same time, he himself constructs a transcendental signified. Rejecting whatever value or outside referent in the process of interpretation is a referent itself and assuming that is no ultimate truth is a transcendental signified as well. It is a referent that we have to keep in mind and refer to all the time and before doing anything. Therefore, how can Derrida ask us to practice what he is not actually doing himself?