Here's what I came up with:
|I use a sheet of 220 grit Wet or Dry sandpaper.|
|The grit is very fine and holds the fabric so that it doesn't shift.|
I use a ruler and a mechanical pencil. The mechanical pencil makes a very fine line and it is easy to get the line right on the edge of the ruler.
|The lines are all straight, the fabric hasn't shifted and it hasn't stretched along the bias.|