Necchi,
I was talking initially about the profile of the forearms as viewed from the top, through a horizontal section. If you are asking about the line of the top of the forearms where it wraps into the barrel, that is a little complicated. Ideally, that line should be below the centerline of the barrel (about the 40% level from the bottom of the side flat) for most of the length of the barrel. As the that line starts at or above center line on both sides of the stock, that line must taper at varying rates. Typically, I terminate the nose piece at about 40% above the bottom of the side flat on both sides of the muzzle. On the lock side, that line runs from the pan at about center line of the barrel to the muzzle in a fairly even taper. On the opposite side of the barrel, you have a roughly parabolic taper from the top of the barrel at the breech to approximately center line of the barrel before you reach the rear entry and then tapering very gently to the muzzle. With the barrel out of the stock, the line of the top of the forearm will appear to be the same on both sides of the barrel channel from in front of the lock panels to the muzzle. By appear, I mean without a real close examination. The fact of the matter is that the two sides of the barrel channel are never the same if the barrel is inlet and the stock shaped by hand. That line will waver up and down trending down to the muzzle. The originals are like this. You have to employ some type of jig using sand paper to get them really close. The original makers wouldn't have done that, they would have just evened the line out with a file if they bothered at all, which it would appear they usually didn't. I will use a mill file or small sanding block to take the fine edge off the sharp sides of the barrel channel before final assembly and finishing. Usually, this is to get rid of small chip outs and irregularities. I can also use it to smooth out that top line some. However, given the warping of the forearm without the barrel to hold it straight, and irregularities in the inletting of the barrel, and irregularities in the thickness of the sidewalls of the barrel channel, it is next to impossible to have a perfectly smooth line. You have to remember that we are talking in generalities with regard to stock architecture. There is some art to this and a good eye is important to make things look right, even if they aren't, and they usually aren't.
I hope you are not still confused although I wouldn't bet the farm on it. It is pretty hard to explain this stuff with just words.