Making the string follow the line of the rafters.
This is very a critical point the construction: You have to look at it from many angles, because wood is not always true and straight. In most every case where your doing "point finding" you have to split the different between several separate measures. You can't relay on one measure, because wood is not straight. The problem is finding one point based on many points, all of which do not line up. It's not like steel. The ends of the rafter line will always be waving to some measurable degree. The wood was not pressed into true geometrical shapes. Nature makes wood crooked. Man tries to straighten it. Futzing it required. I've seen many bad results based on the assumption that wood is straight and true. You must feel the wood, Luke.
For you tech heads: it's a fuzzy way of doing linear regression. Fitting a straight line to bunch of points and other measures that don't fall on a straight line, and than finding the intersection of that line with another fuzzy based straight line, giving you one point that is at the end of the geometric valley, as you define it. The whole roof line depends on this point. You need a clear image, in your head, of what this point is as to carry you forward in the rest of the roof framing.