One is free to use any tool to attempt to understand the King James, but the King James is the final authority.
Do you have any examples you would like to share with us, where the King James was in error?
As you know, I am a strong KJV advocate. But it is not free of textual or translational errors.
As an example of the former, the most notable is the so-called "Johannine comma," in I John 5:7-8. None of the Byzantine manuscripts contain this phrase. It is found only in a handful of very late Western manuscripts and has been pretty convincingly traced to a marginal note of a medieval Catholic manuscript -- from whence it was put back into certain Greek manuscripts in the West.
The entire argument for the authority of the Textus Receptus is based on the fact that it reflects the Byzantine textual tradition, where Greek was used without interruption from the time of the Apostles down to Erasmus's time (and of course is still used today.) The tremendous unanimity of the hundreds of manuscripts divided by long distances and times in the Byzantine textual tradition reflects extremely careful copying and, I believe, the preserving action of the Holy Spirit. (This is pretty close attention to detail regarding the exact words of Scripture for a Church which some believe is only concerned with the general spiritual meaning of the Scriptures.)
The Johannine comma is not incorrect in its theology, but it is a pretty clear late insertion into the text.
Mistranslations are often a matter of opinion. Orthodox Christians would prefer that the ambiguous statement that appears three times in the NT, "ti emoi kai soi", be translated more literally. It literally means "What to me and to thee?"
The KJV translated it in both the case of the Gadarene demoniac and in Christ's words to his mother at Cana as "what have I to do with thee?"
The Catholic Douay-Rheims takes the same ambiguity found in Latin, "Quid mihi et tibi," and interprets it in each passage. For the Gadarene demoniac, it says "what have I to do with Thee?" And in Christ's use of it, the D-R translates the same phrase as "what is that to me and to thee?"
Keep in mind that in the passage in St. Mark, the demoniac runs and does homage to Christ before saying "ti emoi kai soi?" The demons are, in essence, saying to Christ: "we know that you are the Son of God -- and we're powerful spirits ourselves. What does it matter to you what we do with this lowly human?"
If "ti emoi kai soi?" were a put-down statement that basically said "shut up, I have nothing to do with you," (as many Protestants believe that Christ was saying to his mother) then one would not expect the demoniac to do homage (or "worship", as the KJV says -- the Latin says "et adoravit eum" -- adoration is strictly limited to what is due God) to Christ while he says it.
So the Douay-Rheims does capture the essence of what the passage means in the case of Christ speaking to his mother, but something is lost of the ambiguity. Likewise, the KJV misses the point somewhat, I think, in *both* passages.
These, again, are matters of interpretation. It is not possible or desirable always to have an exact equivalence between a Greek word or phrase and an English word or phrase. I recently discussed the "he who loses his life will save it... what shall a man give in exchange for his soul" passage, in which "psyche" is correctly translated into two different English words, depending on context.
Again, I am a great advocate of preserving and using the KJV. I have a KJV Gospel open beside me on a reading stand, a KJV Bible to my right on my desk, and a KJV with Textus Receptus Greek interlinear in front of me. Finding fault with the KJV is sort of like (if you will forgive the inadequate analogy) me critiquing Tiger Woods' golf game or Michael Jordan's jump-shot. The KJV translators are the giants -- we are the midgets who sit on their shoulders in the English-speaking world.
Mat 5:18 For verily I say vnto you, Till heauen and earth passe, one iote or one title, shall in no wise passe from the law, till all be fulfilled.To suggest that the KJV is somehow insuperable would invalidate the nineteenth-century revision of it that we use since the KJV translators were long dead when our revised KJV was edited. And if we claimed that the 1611 was insuperable, we would have invested it with an authority equalling or exceeding the original monographs. Naturally, these positions are impossible to hold honestly. No translation is perfect as the KJV translators made perfectly clear in the Translator's Notes. But that is certainly not to say that all translations are equal.
Mat 5:19 Whosoeuer therfore shall breake one of these least commaundements, and shall teach men so, he shall be called the least in the kingdome of heauen: but whosoeuer shall doe, and teach them, the same shall be called great in the kingdome of heauen.
Mat 5:20 For I say vnto you, That except your righteousnesse shall exceede the righteousnesse of the Scribes and Pharisees, yee shall in no case enter into the kingdome of heauen.
Mat 5:21 Yee haue heard, that it was saide by them of old time, Thou shalt not kill: and, Whosoeuer shall kill, shalbe in danger of the iudgement.
Mat 5:22 But I say vnto you, that whosoeuer is angry with his brother without a cause, shall be in danger of the Iudgement: and whosoeuer shall say to his brother, Racha, shal be in danger of the counsell: but whosoeuer shall say, Thou foole, shalbe in danger of hell fire.