I've been using an old Imperial, similar to this one:
https://www.ebay.com/itm/18551982762...Cclp%3A2047675
I just learned something this past year.
I was having a hard time stopping a new double flare from leaking. After googling, I found out if you don't quite finish the second flare, when you tighten the flare nut, it will "finish" itself. I.E. the flare will finish forming as you tighten it against the seat. Presto! After making several flares, including making a new line when it got too short, using this technique, it sealed on the first try.
Maybe this is common info, but I thought I'd mention it.