There is a lot of questionable information relating to the installation of non-DIY mini split systems. Here is a great video that I believe shows the process to do a great install.
I have watch this guy before, seems very knowledgeable and makes it easy to understand. I noticed he extends the copper tube a little farther then what the specs say in my mini split instructions, he does about a 1/8".
His flare is about the same diameter as the ID of the nut. And that's what you want. I will say that a faulty flare is your most common problem for mini split failure. The proper torque is important also. That gauge is pretty cool though, it defiantly will tell you from good to bad