Guys,
The way I cut a straight line for the beginning of a forestock molding (or TRY to cut it straight). I lay out the first line with pencil using a combination of a long straight edge and dividers - easy so far. Next, I cut a shallow groove very carefully along the pencil line using a hand held V-gouge. Even with my best effort, there will be a bit of variation in the straightness of the line. To finish and straighten the cut I use a small 3-corner file. I carefully position one corner of the file in the shallow groove, then work the file along the length of the groove. The file will bring the groove to final depth and, at the same time, straighten out the slight variations in the original V-gouge cut line.
If a second parallel line is in the forestock molding, I use a coarse cut skip-line checkering tool.
Jim