I find it a little interesting that you claim not to get any feedback on this board, yet I know that I've personally responded to many of your posts. There's no obligation for anyone to reply to anything, you know, and some of us are too busy to come here often. You're welcome.
I just flat iron approximately the width of my iron. It's somewhat arbitrary, but I overlap slightly into the previously treated hair on purpose--it's impossible to measure precisely. By the time you're flat ironing, your hair should be dry. It doesn't matter if you get the solution on the other hair--it's only activated by the high heat of the iron. I made a predetermined decision to retouch approximately 1 inch of hair because the hair grows ~1/4 inch per month, and I want to overlap a bit into the "old" hair anyway.
In other words, I shoot for ~1", but if I go over, I don't worry about it.