Septuagint can be translated as 72, which, of course, refers to the 72 Jewish scholars.
I often thought that the original NT was written in Hebrew, as most of the early Christians were Jews.
I don't use the Septuagint myself. I use the Ancient Greek text for NT and the Tanakh for the OT. It was around before Jesus even came.
As you can probably tell, I am not a big fan of the KJV. I have found too many "mistakes" in it.
Israel was conquered by the Romans & the Israelites and Greeks were associates with each other. There are some evidence of Jesus speaking in Greek to Israelites like when he changed Simon name to Peter, which is "Petros" in Greek meaning stone while the following verse mentions "Petra" which means rock. Even Jewish historian Flavius Josephus who was around for the 1st century said that it was common for Jews to know Greek.
Septuagint is Greek bc they were the conquerers in Alexandria of Egypt at the "supposed" time of the "Septuagint". What you use are the exact things the KJV has only that is ofc in English. Unless you mean you actually read Greek and Hebrew whch I wouldn't recommend. And I believe there are no errors in the KJV so if you feel there are you can post them and I'll try to answer em.
1 Peter 3:15 - But sanctify the Lord God in your hearts: and be ready always to give an answer to every man that asketh you a reason of the hope that is in you with meekness and fear: