Hi lil.D - I didn’t go to a barber before the treatment - I just shaved my full beard as normal. I used the natural line where the skin under my mouth curves downwards onto my neck - if that makes sense. My overall goal is to have my full beard treated - but right now I am getting my chest done and can’t afford the full beard with it. But I can afford the neck, so I’ve stared with that for now. There is definitely an obvious line where the hair has been removed on my neck (less obvious as regrowth happens) but I shave everyday for my job anyway, so the line isn’t really that noticeable. And ultimately once I have my full beard treated, there will be a reduction all over, so there won’t be any obvious ‘lines’ anywhere.
As for your own situation - if your intention is just to treat your neck, I would suggest that after the laser treatments you might consider electrolysis around the ‘line’ area to blend it in with your beard. It’s just a suggestion so that it will look natural for you. If you have any more questions, just ask.